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HAMMER(8) System Manager's Manual HAMMER(8)

hammer — HAMMER file system utility

hammer -h

hammer [-2ABFqrvXy] [-b bandwidth] [-C cachesize[:readahead]] [-R restrictcmd] [-T restrictpath] [-c cyclefile] [-e scoreboardfile] [-f blkdevs] [-i delay] [-p ssh-port] [-S splitsize] [-t seconds] [-m memlimit] command [argument ...]

This manual page documents the hammer utility which provides miscellaneous functions related to managing a HAMMER file system. For a general introduction to the HAMMER file system, its features, and examples on how to set up and maintain one, see HAMMER(5).

The options are as follows:

Tell the mirror commands to use a 2-way protocol, which allows automatic negotiation of transaction id ranges. This option is automatically enabled by the mirror-copy command.
Make per PFS commands perform on all PFSs if possible. If the command supports this option, it targets all PFSs of the HAMMER filesystem that the filesystem argument (of that command) belongs to. Currently rebalance, reblock, reblock-btree, reblock-inodes, reblock-dirs and reblock-data commands support this option. If the command does not support this option, it does nothing.
Bulk transfer. Mirror-stream will not attempt to break-up large initial bulk transfers into smaller pieces. This can save time but if the link is lost in the middle of the initial bulk transfer you will have to start over from scratch. For more information see the -S option.
bandwidth
Specify a bandwidth limit in bytes per second for mirroring streams. This option is typically used to prevent batch mirroring operations from loading down the machine. The bandwidth may be suffixed with k, m, or g to specify values in kilobytes, megabytes, and gigabytes per second. If no suffix is specified, bytes per second is assumed.

Unfortunately this is only applicable to the pre-compression bandwidth when compression is used, so a better solution would probably be to use a ipfw(8) pipe or a pf(4) queue.

cachesize[:readahead]
Set the memory cache size for any raw I/O. The default is 16MB. A suffix of k for kilobytes and m for megabytes is allowed, else the cache size is specified in bytes.

The read-behind/read-ahead defaults to 4 HAMMER blocks.

This option is typically only used with diagnostic commands as kernel-supported commands will use the kernel's buffer cache.

restrictcmd
This option is used by hammer ssh-remote to restrict the command later on in the argument list. Multiple commands may be specified, separated by a comma (all one argument).
restrictpath
This option is used by hammer ssh-remote to restrict the filesystem path specified later on in the argument list.
cyclefile
When pruning, rebalancing or reblocking you can tell the utility to start at the object id stored in the specified file. If the file does not exist hammer will start at the beginning. If hammer is told to run for a specific period of time (-t) and is unable to complete the operation it will write out the current object id so the next run can pick up where it left off. If hammer runs to completion it will delete cyclefile.
scoreboardfile
Update scoreboard file with progress, primarily used by mirror-stream.
Force operation. E.g. cleanup will not check that time period has elapsed if this option is given.
blkdevs
Specify the volumes making up a HAMMER file system. Blkdevs is a colon-separated list of devices, each specifying a HAMMER volume.
Show usage.
delay
Specify delay in seconds for mirror-read-stream. When maintaining a streaming mirroring this option specifies the minimum delay after a batch ends before the next batch is allowed to start. The default is five seconds.
memlimit
Specify the maximum amount of memory hammer will allocate during a dedup pass. Specify a suffix 'm', 'g', or 't' for megabytes, gigabytes, or terabytes. By default hammer will allocate up to 1G of ram to hold CRC/SHA tables while running dedup. When the limit is reached the dedup code restricts the range of CRCs to keep memory use within bounds and runs multiple passes as necessary until the entire filesystem has been deduped.
ssh-port
Pass the -p ssh-port option to ssh(1) when using a remote specification for the source and/or destination.
Decrease verboseness. May be specified multiple times.
Specify recursion for those commands which support it.
splitsize
Specify the bulk splitup size in bytes for mirroring streams. When a mirror-stream is first started hammer will do an initial run-through of the data to calculate good transaction ids to cut up the bulk transfers, creating restart points in case the stream is interrupted. If we don't do this and the stream is interrupted it might have to start all over again. The default is a splitsize of 4GB.

At the moment the run-through is disk-bandwidth-heavy but some future version will limit the run-through to just the B-Tree records and not the record data.

The splitsize may be suffixed with k, m, or g to specify values in kilobytes, megabytes, or gigabytes. If no suffix is specified, bytes is assumed.

When mirroring very large filesystems the minimum recommended split size is 4GB. A small split size may wind up generating a great deal of overhead but very little actual incremental data and is not recommended.

seconds
Specify timeout in seconds. When pruning, rebalancing, reblocking or mirror-reading you can tell the utility to stop after a certain period of time. A value of 0 means unlimited. This option is used along with the -c cyclefile option to prune, rebalance or reblock incrementally.
Increase verboseness. May be specified multiple times.
Enable compression for any remote ssh specifications. This option is typically used with the mirroring directives.
Force “yes” for interactive questions.

The commands are as follows:

filesystem [quick]
Generate a guaranteed, formal 64-bit transaction id representing the current state of the specified HAMMER file system. The file system will be synced to the media.

If the quick keyword is specified the file system will be soft-synced, meaning that a crash might still undo the state of the file system as of the transaction id returned but any new modifications will occur after the returned transaction id as expected.

This operation does not create a snapshot. It is meant to be used to track temporary fine-grained changes to a subset of files and will only remain valid for ‘@@’ access purposes for the prune-min period configured for the PFS. If you desire a real snapshot then the snapq directive may be what you are looking for.

[interval]
Output HAMMER B-Tree statistics until interrupted. Pause interval seconds between each display. The default interval is one second.
[interval]
Output HAMMER I/O statistics until interrupted. Pause interval seconds between each display. The default interval is one second.
[interval]
Output HAMMER B-Tree and I/O statistics until interrupted. Pause interval seconds between each display. The default interval is one second.
[@offset[,length]] path[@offset[,length]] ...
Show the modification history for inode and data of HAMMER files. If offset is given history is shown for data block at given offset, otherwise history is shown for inode. If -v is specified length data bytes at given offset are dumped for each version, default is 32.

For each path this directive shows object id and sync status, and for each object version it shows transaction id and time stamp. Files has to exist for this directive to be applicable, to track inodes which has been deleted or renamed see undo(1).

Different offset and length can be used for each path by specifying offset and length for each path.

Dump the blockmap for the file system. The HAMMER blockmap is two-layer blockmap representing the maximum possible file system size of 1 Exabyte. Needless to say the second layer is only present for blocks which exist. HAMMER's blockmap represents 8-Megabyte blocks, called big-blocks. Each big-block has an append point, a free byte count, and a typed zone id which allows content to be reverse engineered to some degree.

In HAMMER allocations are essentially appended to a selected big-block using the append offset and deducted from the free byte count. When space is freed the free byte count is adjusted but HAMMER does not track holes in big-blocks for reallocation. A big-block must be completely freed, either through normal file system operations or through reblocking, before it can be reused.

Data blocks can be shared by deducting the space used from the free byte count for each shared references. This means the free byte count can legally go negative.

This command needs the -f blkdevs option.

Check the blockmap allocation count. hammer will scan the freemap, B-Tree, UNDO/REDO FIFO, then collect allocation information, and construct a blockmap in-memory. It will then check that blockmap against the on-disk blockmap.

This command needs the -f blkdevs option.

[localization[:object_id[:rec_type[:key[:create_tid]]]]]
Dump the B-Tree. By default this command will validate all B-Tree linkages and CRCs, including data CRCs, and will report the most verbose information it can dig up. Any errors will show up with a ‘B’ in column 1 along with various other error flags.

If you specify localization or localization:object_id or localization:object_id:rec_type or localization:object_id:rec_type:key or localization:object_id:rec_type:key:create_tid the dump will search for the key printing nodes as it recurses down, and then will iterate forwards. These fields are specified in HEX. Note that the PFS id is the top 16 bits of the 32-bit localization field so PFS#1 would be 00010000.

If you use -q the command will report less information about the inode contents.

If you use -qq the command will not report the content of the inode or other typed data at all.

If you use -qqq the command will not report volume header information, big-block fill ratios, mirror transaction ids, or report or check data CRCs. B-Tree CRCs and linkages are still checked.

This command needs the -f blkdevs option.

(HAMMER VERSION 4+) Dump the UNDO/REDO map.

This command needs the -f blkdevs option.

command targetdir
Used in a ssh authorized_keys line such as command="/sbin/hammer ssh-remote mirror-read /fubarmount" ... to allow mirror-read or mirror-write access to a particular subdirectory tree. This way you do not have to give shell access to the remote box. hammer will obtain the original command line from the SSH_ORIGINAL_COMMAND environment variable, validate it against the restriction, and then re-exec hammer with the validated arguments.

The remote hammer command does not allow the -c or -f options to be passed in.

targetdir [full|quick]
Recover data from a corrupted HAMMER filesystem. This is a low level command which operates on the filesystem image and attempts to locate and recover files from a corrupted filesystem. The entire image is scanned linearly looking for B-Tree nodes. Any node found which passes its CRC test is scanned for file, inode, and directory fragments and the target directory is populated with the resulting data. files and directories in the target directory are initially named after the object id and are renamed as fragmentary information is processed.

This command keeps track of filename/object_id translations and may eat a considerably amount of memory while operating.

This command is literally the last line of defense when it comes to recovering data from a dead filesystem.

If the full keyword is specified, this command scans all the big-blocks within the entire image, regardless of whether big-blocks contain active data or meta-data. By default, it linearly scans only up to the last big-block that contains active data or meta-data, if such big-block is detectable.

If the quick keyword is specified, this command only scans big-blocks that contain B-Tree nodes and associated records, without linearly scanning the entire image. This may speed up the recovery process provided that big-blocks for B-Tree nodes are not corrupted.

This command needs the -f blkdevs option.

Strip HAMMER filesystem volume header and other meta-data by overwriting them with irrelevant data. HAMMER volumes need to be unmounted.

This is a fast way to make HAMMER filesystem unmountable and unrecoverable. After running this command, HAMMER filesystem data is no longer recoverable using recover command, although the data still exists within the volumes. As safety measure the -y flag have no effect on this directive.

This command needs the -f blkdevs option.

filename
Generate a HAMMER 64-bit directory hash for the specified file name, using the original directory hash algorithm in version 1 of the file system. The low 32 bits are used as an iterator for hash collisions and will be output as 0.
filename
Generate a HAMMER 64-bit directory hash for the specified file name, using the new directory hash algorithm in version 2 of the file system. The low 32 bits are still used as an iterator but will start out containing part of the hash key.
filename
Generate the top 32 bits of a HAMMER 64 bit directory hash for the specified file name.
dirpath ...
Show extended information about all HAMMER file systems mounted in the system or the one mounted in dirpath when this argument is specified.

The information is divided into sections:

Volume identification
General information, like the label of the HAMMER filesystem, the number of volumes it contains, the FSID, and the HAMMER version being used.
Big-block information
Big-block statistics, such as total, used, reserved and free big-blocks.
Space information
Information about space used on the filesystem. Currently total size, used, reserved and free space are displayed.
PFS information
Basic information about the PFSs currently present on a HAMMER filesystem.

“PFS ID” is the ID of the PFS, with 0 being the root PFS. “Snaps” is the current snapshot count on the PFS. “Mounted on” displays the mount point of the PFS is currently mounted on (if any).

[filesystem ...]
This is a meta-command which executes snapshot, prune, rebalance, dedup and reblock commands on the specified HAMMER file systems. If no filesystem is specified this command will clean-up all HAMMER file systems in use, including PFS's. To do this it will scan all HAMMER and null mounts, extract PFS id's, and clean-up each PFS found.

This command will access a snapshots directory and a configuration file for each filesystem, creating them if necessary.

HAMMER version 2-
The configuration file is config in the snapshots directory which defaults to <pfs>/snapshots.
HAMMER version 3+
The configuration file is saved in file system meta-data, see hammer config. The snapshots directory defaults to /var/hammer/<pfs> (/var/hammer/root for root mount).

The format of the configuration file is:

snapshots  <period> <retention-time> [any]
prune      <period> <max-runtime>
rebalance  <period> <max-runtime>
dedup      <period> <max-runtime>
reblock    <period> <max-runtime>
recopy     <period> <max-runtime>

Defaults are:

snapshots  1d 60d  # 0d 0d  for PFS /tmp, /var/tmp, /usr/obj
prune      1d 5m
rebalance  1d 5m
#dedup      1d 5m  # not enabled by default
reblock    1d 5m
recopy     30d 10m

Time is given with a suffix of d, h, m or s meaning day, hour, minute and second.

If the snapshots directive has a period of 0 and a retention time of 0 then snapshot generation is disabled, removal of old snapshots are disabled, and prunes will use prune-everything.

If the snapshots directive has a period of 0 but a non-zero retention time then this command will not create any new snapshots but will remove old snapshots it finds based on the retention time. This form should be used on PFS masters where you are generating your own snapshot softlinks manually and on PFS slaves when all you wish to do is prune away existing snapshots inherited via the mirroring stream.

By default only snapshots in the form ‘snap-yyyymmdd[-HHMM]’ are processed. If the any directive is specified as a third argument on the snapshots config line then any softlink of the form ‘*-yyyymmdd[-HHMM]’ or ‘*.yyyymmdd[-HHMM]’ will be processed.

A period of 0 for prune, rebalance, dedup, reblock or recopy disables the directive. A max-runtime of 0 means unlimited.

If period hasn't passed since the previous cleanup run nothing is done. For example a day has passed when midnight is passed (localtime). If the -F flag is given the period is ignored. By default, DragonFly is set up to run hammer cleanup nightly via periodic(8).

The default configuration file will create a daily snapshot, do a daily pruning, rebalancing, deduping and reblocking run and a monthly recopy run. Reblocking is defragmentation with a level of 95%, and recopy is full defragmentation.

By default prune, dedup and rebalance operations are time limited to 5 minutes, and reblock operations to a bit over 5 minutes, and recopy operations to a bit over 10 minutes. Reblocking and recopy runs are each broken down into four separate functions: B-Tree, inodes, dirs and data. Each function is time limited to the time given in the configuration file, but the B-Tree, inodes and dirs functions usually does not take very long time, full defragmentation is always used for these three functions. Also note that this directive will by default disable snapshots on the following PFS's: /tmp, /var/tmp and /usr/obj.

The defaults may be adjusted by modifying the configuration file. The pruning and reblocking commands automatically maintain a cyclefile for incremental operation. If you interrupt (^C) the program the cyclefile will be updated, but a sub-command may continue to run in the background for a few seconds until the HAMMER ioctl detects the interrupt. The snapshots PFS option can be set to use another location for the snapshots directory.

Work on this command is still in progress. Expected additions: An ability to remove snapshots dynamically as the file system becomes full.

This command will terminate all active cleanup processes.
[filesystem [configfile]]
(HAMMER VERSION 3+) Show or change configuration for filesystem. If zero or one arguments are specified this function dumps the current configuration file to stdout. Zero arguments specifies the PFS containing the current directory. This configuration file is stored in file system meta-data. If two arguments are specified this function installs a new configuration file configfile.

In HAMMER versions less than 3 the configuration file is by default stored in <pfs>/snapshots/config, but in all later versions the configuration file is stored in file system meta-data.

[filesystem]
(HAMMER VERSION 3+) Edit the configuration file and reinstall into file system meta-data when done. Zero arguments specifies the PFS containing the current directory.
device filesystem
Add volume device to filesystem. This will format device and add all of its space to filesystem. A HAMMER file system can use up to 256 volumes.

NOTE! All existing data contained on device will be destroyed by this operation! If device contains a valid HAMMER file system, formatting will be denied. You can overcome this sanity check by using dd(1) to erase the beginning sectors of the device.

Remember that you have to specify device, together with any other device that make up the file system, colon-separated to /etc/fstab and mount_hammer(8). If filesystem is root file system, also remember to add device to vfs.root.mountfrom in /boot/loader.conf, see loader(8).

device filesystem
Remove volume device from filesystem.

Remember that you have to remove device from the colon-separated list in /etc/fstab and mount_hammer(8). If filesystem is root file system, also remember to remove device from vfs.root.mountfrom in /boot/loader.conf, see loader(8).

It is not possible to remove the root-volume as it contains filesystem meta data such as HAMMER's layer1 blockmap and UNDO/REDO FIFO.

This command may reblock filesystem before it attempts to remove the volume if the volume is not empty.

filesystem
List the volumes that make up filesystem. If -v is specified the command shows volume number for each volume as well as root-volume information.
filesystem
List the volumes that make up filesystem in blkdevs format.
[filesystem] snapshot-dir
 
filesystem snapshot-dir [note]
Take a snapshot of the file system either explicitly given by filesystem or implicitly derived from the snapshot-dir argument and creates a symlink in the directory provided by snapshot-dir pointing to the snapshot. If snapshot-dir is not a directory, it is assumed to be a format string passed to strftime(3) with the current time as parameter. If snapshot-dir refers to an existing directory, a default format string of ‘snap-%Y%m%d-%H%M’ is assumed and used as name for the newly created symlink.

Snapshot is a per PFS operation, so each PFS in a HAMMER file system have to be snapshot separately.

Example, assuming that /mysnapshots is on file system / and that /obj and /usr are file systems on their own, the following invocations:

hammer snapshot /mysnapshots

hammer snapshot /mysnapshots/%Y-%m-%d

hammer snapshot /obj /mysnapshots/obj-%Y-%m-%d

hammer snapshot /usr /my/snaps/usr "note"

Would create symlinks similar to:

/mysnapshots/snap-20080627-1210 -> /@@0x10d2cd05b7270d16

/mysnapshots/2008-06-27 -> /@@0x10d2cd05b7270d16

/mysnapshots/obj-2008-06-27 -> /obj@@0x10d2cd05b7270d16

/my/snaps/usr/snap-20080627-1210 -> /usr@@0x10d2cd05b7270d16

When run on a HAMMER version 3+ file system the snapshot is also recorded in file system meta-data along with the optional note. See the snapls directive.

path [note]
(HAMMER VERSION 3+) Create a snapshot for the PFS containing path and create a snapshot softlink. If the path specified is a directory a standard snapshot softlink will be created in the directory. The snapshot softlink points to the base of the mounted PFS.
path [note]
(HAMMER VERSION 3+) Create a snapshot for the PFS containing path and create a snapshot softlink. If the path specified is a directory a standard snapshot softlink will be created in the directory. The snapshot softlink points into the directory it is contained in.
dir [note]
(HAMMER VERSION 3+) Create a snapshot for the PFS containing the specified directory but do not create a softlink. Instead output a path which can be used to access the directory via the snapshot.

An absolute or relative path may be specified. The path will be used as-is as a prefix in the path output to stdout. As with the other snap and snapshot directives the snapshot transaction id will be registered in the file system meta-data.

path ...
 
transaction_id ...
 
filesystem transaction_id ...
(HAMMER VERSION 3+) Remove a snapshot given its softlink or transaction id. If specifying a transaction id the snapshot is removed from file system meta-data but you are responsible for removing any related softlinks.

If a softlink path is specified the filesystem and transaction id is derived from the contents of the softlink. If just a transaction id is specified it is assumed to be a snapshot in the HAMMER filesystem you are currently chdir'd into. You can also specify the filesystem and transaction id explicitly.

[path ...]
(HAMMER VERSION 3+) Dump the snapshot meta-data for PFSs containing each path listing all available snapshots and their notes. If no arguments are specified snapshots for the PFS containing the current directory are listed. This is the definitive list of snapshots for the file system.
softlink-dir
Prune the file system based on previously created snapshot softlinks. Pruning is the act of deleting file system history. The prune command will delete file system history such that the file system state is retained for the given snapshots, and all history after the latest snapshot. By setting the per PFS parameter prune-min, history is guaranteed to be saved at least this time interval. All other history is deleted.

The target directory is expected to contain softlinks pointing to snapshots of the file systems you wish to retain. The directory is scanned non-recursively and the mount points and transaction ids stored in the softlinks are extracted and sorted. The file system is then explicitly pruned according to what is found. Cleaning out portions of the file system is as simple as removing a snapshot softlink and then running the prune command.

As a safety measure pruning only occurs if one or more softlinks are found containing the ‘@@’ snapshot id extension. Currently the scanned softlink directory must contain softlinks pointing to a single HAMMER mount. The softlinks may specify absolute or relative paths. Softlinks must use 20-character ‘@@0x%016llx’ transaction ids, as might be returned from hammer synctid filesystem.

Pruning is a per PFS operation, so each PFS in a HAMMER file system have to be pruned separately.

Note that pruning a file system may not immediately free-up space, though typically some space will be freed if a large number of records are pruned out. The file system must be reblocked to completely recover all available space.

Example, lets say your that you didn't set prune-min, and snapshot directory contains the following links:

lrwxr-xr-x  1 root  wheel  29 May 31 17:57 snap1 ->
/usr/obj/@@0x10d2cd05b7270d16

lrwxr-xr-x  1 root  wheel  29 May 31 17:58 snap2 ->
/usr/obj/@@0x10d2cd13f3fde98f

lrwxr-xr-x  1 root  wheel  29 May 31 17:59 snap3 ->
/usr/obj/@@0x10d2cd222adee364

If you were to run the prune command on this directory, then the HAMMER /usr/obj mount will be pruned to retain the above three snapshots. In addition, history for modifications made to the file system older than the oldest snapshot will be destroyed and history for potentially fine-grained modifications made to the file system more recently than the most recent snapshot will be retained.

If you then delete the snap2 softlink and rerun the prune command, history for modifications pertaining to that snapshot would be destroyed.

In HAMMER file system versions 3+ this command also scans the snapshots stored in the file system meta-data and includes them in the prune.

filesystem
Remove all historical records from filesystem. Use this directive with caution on PFSs where you intend to use history.

This command does not remove snapshot softlinks but will delete all snapshots recorded in file system meta-data (for file system version 3+). The user is responsible for deleting any softlinks.

Pruning is a per PFS operation, so each PFS in a HAMMER file system have to be pruned separately.

filesystem [saturation_percentage]
Rebalance the B-Tree, nodes with small number of elements will be combined and element counts will be smoothed out between nodes.

The saturation percentage is between 50% and 100%. The default is 85% (the ‘%’ suffix is not needed).

Rebalancing is a per PFS operation, so each PFS in a HAMMER file system have to be rebalanced separately.

filesystem
(HAMMER VERSION 5+) Perform offline (post-process) deduplication. Deduplication occurs at the block level, currently only data blocks of the same size can be deduped, metadata blocks can not. The hash function used for comparing data blocks is CRC-32 (CRCs are computed anyways as part of HAMMER data integrity features, so there's no additional overhead). Since CRC is a weak hash function a byte-by-byte comparison is done before actual deduping. In case of a CRC collision (two data blocks have the same CRC but different contents) the checksum is upgraded to SHA-256.

Currently HAMMER reblocker may partially blow up (re-expand) dedup (reblocker's normal operation is to reallocate every record, so it's possible for deduped blocks to be re-expanded back).

Deduplication is a per PFS operation, so each PFS in a HAMMER file system have to be deduped separately. This also means that if you have duplicated data in two different PFSs that data won't be deduped, however the addition of such feature is planned.

The -m memlimit option should be used to limit memory use during the dedup run if the default 1G limit is too much for the machine.

filesystem
Shows potential space savings (simulated dedup ratio) one can get after running dedup command. If the estimated dedup ratio is greater than 1.00 you will see dedup space savings. Remember that this is an estimated number, in practice real dedup ratio will be slightly smaller because of HAMMER big-block underflows, B-Tree locking issues and other factors.

Note that deduplication currently works only on bulk data so if you try to run dedup-simulate or dedup commands on a PFS that contains metadata only (directory entries, softlinks) you will get a 0.00 dedup ratio.

The -m memlimit option should be used to limit memory use during the dedup run if the default 1G limit is too much for the machine.

filesystem [fill_percentage]
 
filesystem [fill_percentage]
 
filesystem [fill_percentage]
 
filesystem [fill_percentage]
 
filesystem [fill_percentage]
Attempt to defragment and free space for reuse by reblocking a live HAMMER file system. Big-blocks cannot be reused by HAMMER until they are completely free. This command also has the effect of reordering all elements, effectively defragmenting the file system.

The default fill percentage is 100% and will cause the file system to be completely defragmented. All specified element types will be reallocated and rewritten. If you wish to quickly free up space instead try specifying a smaller fill percentage, such as 90% or 80% (the ‘%’ suffix is not needed).

Since this command may rewrite the entire contents of the disk it is best to do it incrementally from a cron(8) job along with the -c cyclefile and -t seconds options to limit the run time. The file system would thus be defragmented over long period of time.

It is recommended that separate invocations be used for each data type. B-Tree nodes, inodes, and directories are typically the most important elements needing defragmentation. Data can be defragmented over a longer period of time.

Reblocking is a per PFS operation, so each PFS in a HAMMER file system have to be reblocked separately.

dirpath ...
Retrieve the mirroring configuration parameters for the specified HAMMER file systems or pseudo-filesystems (PFS's).
dirpath [options]
Create a pseudo-filesystem (PFS) inside a HAMMER file system. Up to 65536 PFSs can be created. Each PFS uses an independent inode numbering space making it suitable for replication.

The pfs-master directive creates a PFS that you can read, write, and use as a mirroring source.

A PFS can only be truly destroyed with the pfs-destroy directive. Removing the softlink will not destroy the underlying PFS.

A PFS can only be created in the root PFS, not in a PFS created by pfs-master or pfs-slave.

It is recommended that dirpath is of the form <fs>/pfs/<name> (i.e. located in PFS directory at root of HAMMER file system).

It is recommended to use a null mount to access a PFS, except for root PFS, for more information see HAMMER(5).

dirpath [options]
Create a pseudo-filesystem (PFS) inside a HAMMER file system. Up to 65536 PFSs can be created. Each PFS uses an independent inode numbering space making it suitable for replication.

The pfs-slave directive creates a PFS that you can use as a mirroring source or target. You will not be able to access a slave PFS until you have completed the first mirroring operation with it as the target (its root directory will not exist until then).

Access to the pfs-slave via the special softlink, as described in the PSEUDO-FILESYSTEM (PFS) NOTES below, allows HAMMER to dynamically modify the snapshot transaction id by returning a dynamic result from readlink(2) calls.

A PFS can only be truly destroyed with the pfs-destroy directive. Removing the softlink will not destroy the underlying PFS.

A PFS can only be created in the root PFS, not in a PFS created by pfs-master or pfs-slave.

It is recommended that dirpath is of the form <fs>/pfs/<name> (i.e. located in PFS directory at root of HAMMER file system).

It is recommended to use a null mount to access a PFS, except for root PFS, for more information see HAMMER(5).

dirpath [options]
Update the configuration parameters for an existing HAMMER file system or pseudo-filesystem. Options that may be specified:
0x16llx
This is the automatic snapshot access starting transaction id for mirroring slaves. This parameter is normally updated automatically by the mirror-write directive.

It is important to note that accessing a mirroring slave with a transaction id greater than the last fully synchronized transaction id can result in an unreliable snapshot since you will be accessing data that is still undergoing synchronization.

Manually modifying this field is dangerous and can result in a broken mirror.

0x16llx
This is the current synchronization point for mirroring slaves. This parameter is normally updated automatically by the mirror-write directive.

Manually modifying this field is dangerous and can result in a broken mirror.

uuid
Set the shared UUID for this file system. All mirrors must have the same shared UUID. For safety purposes the mirror-write directives will refuse to operate on a target with a different shared UUID.

Changing the shared UUID on an existing, non-empty mirroring target, including an empty but not completely pruned target, can lead to corruption of the mirroring target.

uuid
Set the unique UUID for this file system. This UUID should not be used anywhere else, even on exact copies of the file system.
string
Set a descriptive label for this file system.
string
Specify the snapshots directory which hammer cleanup will use to manage this PFS.
HAMMER version 2-
The snapshots directory does not need to be configured for PFS masters and will default to <pfs>/snapshots.

PFS slaves are mirroring slaves so you cannot configure a snapshots directory on the slave itself to be managed by the slave's machine. In fact, the slave will likely have a snapshots sub-directory mirrored from the master, but that directory contains the configuration the master is using for its copy of the file system, not the configuration that we want to use for our slave.

It is recommended that <fs>/var/slaves/<name> be configured for a PFS slave, where <fs> is the base HAMMER file system, and <name> is an appropriate label.

HAMMER version 3+
The snapshots directory does not need to be configured for PFS masters or slaves. The snapshots directory defaults to /var/hammer/<pfs> (/var/hammer/root for root mount).

You can control snapshot retention on your slave independent of the master.

Zero out the snapshots directory path for this PFS.
Nd
 
[Nd/]hh[:mm[:ss]]
Set the minimum fine-grained data retention period. HAMMER always retains fine-grained history up to the most recent snapshot. You can extend the retention period further by specifying a non-zero pruning minimum. Any snapshot softlinks within the retention period are ignored for the purposes of pruning (i.e. the fine grained history is retained). Number of days, hours, minutes and seconds are given as N, hh, mm and ss.

Because the transaction id in the snapshot softlink cannot be used to calculate a timestamp, HAMMER uses the earlier of the st_ctime or st_mtime field of the softlink to determine which snapshots fall within the retention period. Users must be sure to retain one of these two fields when manipulating the softlink.

dirpath
Upgrade a PFS from slave to master operation. The PFS will be rolled back to the current end synchronization transaction id (removing any partial synchronizations), and will then become writable.

WARNING! HAMMER currently supports only single masters and using this command can easily result in file system corruption if you don't know what you are doing.

This directive will refuse to run if any programs have open descriptors in the PFS, including programs chdir'd into the PFS.

dirpath
Downgrade a master PFS from master to slave operation. The PFS becomes read-only and access will be locked to its sync-end-tid.

This directive will refuse to run if any programs have open descriptors in the PFS, including programs chdir'd into the PFS.

dirpath
This permanently destroys a PFS.

This directive will refuse to run if any programs have open descriptors in the PFS, including programs chdir'd into the PFS. As safety measure the -y flag have no effect on this directive.

filesystem [begin-tid]
Generate a mirroring stream to stdout. The stream ends when the transaction id space has been exhausted. filesystem may be a master or slave PFS.
filesystem [begin-tid]
Generate a mirroring stream to stdout. Upon completion the stream is paused until new data is synced to the filesystem, then resumed. Operation continues until the pipe is broken. See the mirror-stream command for more details.
filesystem
Take a mirroring stream on stdin. filesystem must be a slave PFS.

This command will fail if the shared-uuid configuration field for the two file systems do not match. See the mirror-copy command for more details.

If the target PFS does not exist this command will ask you whether you want to create a compatible PFS slave for the target or not.

mirror-dump [header]
A mirror-read can be piped into a mirror-dump to dump an ASCII representation of the mirroring stream. If the keyword header is specified, only the header information is shown.
[[user@]host:]filesystem [[user@]host:]filesystem
This is a shortcut which pipes a mirror-read command to a mirror-write command. If a remote host specification is made the program forks a ssh(1) (or other program as specified by the HAMMER_RSH environment variable) and execs the mirror-read and/or mirror-write on the appropriate host. The source may be a master or slave PFS, and the target must be a slave PFS.

This command also establishes full duplex communication and turns on the 2-way protocol feature (-2) which automatically negotiates transaction id ranges without having to use a cyclefile. If the operation completes successfully the target PFS's sync-end-tid will be updated. Note that you must re-chdir into the target PFS to see the updated information. If you do not you will still be in the previous snapshot.

If the target PFS does not exist this command will ask you whether you want to create a compatible PFS slave for the target or not.

[[user@]host:]filesystem [[user@]host:]filesystem
This is a shortcut which pipes a mirror-read-stream command to a mirror-write command. This command works similarly to mirror-copy but does not exit after the initial mirroring completes. The mirroring operation will resume as changes continue to be made to the source. The command is commonly used with -i delay and -b bandwidth options to keep the mirroring target in sync with the source on a continuing basis.

If the pipe is broken the command will automatically retry after sleeping for a short while. The time slept will be 15 seconds plus the time given in the -i option.

This command also detects the initial-mirroring case and spends some time scanning the B-Tree to find good break points, allowing the initial bulk mirroring operation to be broken down into 4GB pieces. This means that the user can kill and restart the operation and it will not have to start from scratch once it has gotten past the first chunk. The -S option may be used to change the size of pieces and the -B option may be used to disable this feature and perform an initial bulk transfer instead.

filesystem
This command returns the HAMMER file system version for the specified filesystem as well as the range of versions supported in the kernel. The -q option may be used to remove the summary at the end.
filesystem version [force]
Upgrade the HAMMER filesystem to the specified version. Once upgraded a file system may not be downgraded. If you wish to upgrade a file system to a version greater or equal to the work-in-progress (WIP) version number you must specify the force directive. Use of WIP versions should be relegated to testing and may require wiping the file system as development progresses, even though the WIP version might not change.

NOTE! This command operates on the entire HAMMER file system and is not a per PFS operation. All PFS's will be affected.

1
DragonFly 2.0 default version, first HAMMER release.
2
DragonFly 2.3. New directory entry layout. This version is using a new directory hash key.
3
DragonFly 2.5. New snapshot management, using file system meta-data for saving configuration file and snapshots (transaction ids etc.). Also default snapshots directory has changed.
4
DragonFly 2.6 default version. New undo/redo/flush, giving HAMMER a much faster sync and fsync.
5
DragonFly 2.9. Deduplication support.
6
DragonFly 2.9. Directory hash ALG1. Tends to maintain inode number / directory name entry ordering better for files after minor renaming.
7
DragonFly 4.8. Use the ISCSI crc32 algorithm instead of our custom polynomial. This makes it easier to use faster cpu implementation of the crc algorithm. CPU overhead is reduced but performance is unlikely to be impacted a whole lot.

The root of a PFS is not hooked into the primary HAMMER file system as a directory. Instead, HAMMER creates a special softlink called ‘@@PFS%05d’ (exactly 10 characters long) in the primary HAMMER file system. HAMMER then modifies the contents of the softlink as read by readlink(2), and thus what you see with an ls command or if you were to cd into the link. If the PFS is a master the link reflects the current state of the PFS. If the PFS is a slave the link reflects the last completed snapshot, and the contents of the link will change when the next snapshot is completed, and so forth.

The hammer utility employs numerous safeties to reduce user foot-shooting. The mirror-copy directive requires that the target be configured as a slave and that the shared-uuid field of the mirroring source and target match. The pfs-master and pfs-slave directives require that the PFS softlink be created under the main hammer filesystem mount. You may only access PFS softlinks via the main hammer filesystem mount.

There is a limit to the number of vnodes the kernel can cache, and because file buffers are associated with a vnode the related data cache can get blown away when operating on large numbers of files even if the system has sufficient memory to hold the file data.

If you turn on HAMMER's double buffer mode by setting the sysctl(8) node vfs.hammer.double_buffer to 1 HAMMER will cache file data via the block device and copy it into the per-file buffers as needed. The data will be double-cached at least until the buffer cache throws away the file buffer. This mode is typically used in conjunction with swapcache(8) when vm.swapcache.data_enable is turned on in order to prevent unnecessary re-caching of file data due to vnode recycling. The swapcache will save the cached VM pages related to HAMMER's block device (which doesn't recycle unless you umount the filesystem) instead of the cached VM pages backing the file vnodes.

Double buffering is normally desirable when working with large filesystems, particularly when swapcache is used. The swapcache can only back active VM objects, including the block device, and large filesystems often have far more inodes than the kernel can support. In addition, when using this mode, you may wish to reduce the kern.maxvnodes setting for the system to force the system to do less caching of logical file buffers and more caching of device buffers, since the device buffers are backing the logical file buffers.

This upgrade changes the way directory entries are stored. It is possible to upgrade a V1 file system to V2 in place, but directories created prior to the upgrade will continue to use the old layout.

Note that the slave mirroring code in the target kernel had bugs in V1 which can create an incompatible root directory on the slave. Do not mix a HAMMER master created after the upgrade with a HAMMER slave created prior to the upgrade.

Any directories created after upgrading will use a new layout.

This upgrade adds meta-data elements to the B-Tree. It is possible to upgrade a V2 file system to V3 in place. After issuing the upgrade be sure to run a hammer cleanup to perform post-upgrade tasks.

After making this upgrade running a hammer cleanup will move the <pfs>/snapshots directory for each PFS mount into /var/hammer/<pfs>. A HAMMER root mount will migrate /snapshots into /var/hammer/root. Migration occurs only once and only if you have not specified a snapshots directory in the PFS configuration. If you have specified a snapshots directory in the PFS configuration no automatic migration will occur.

For slaves, if you desire, you can migrate your snapshots config to the new location manually and then clear the snapshot directory configuration in the slave PFS. The new snapshots hierarchy is designed to work with both master and slave PFSs equally well.

In addition, the old config file will be moved to file system meta-data, editable via the new hammer viconfig directive. The old config file will be deleted. Migration occurs only once.

The V3 file system has new snap* directives for creating snapshots. All snapshot directives, including the original, will create meta-data entries for the snapshots and the pruning code will automatically incorporate these entries into its list and expire them the same way it expires softlinks. If you by accident blow away your snapshot softlinks you can use the snapls directive to get a definitive list from the file system meta-data and regenerate them from that list.

WARNING! If you are using hammer to backup file systems your scripts may be using the synctid directive to generate transaction ids. This directive does not create a snapshot. You will have to modify your scripts to use the snapq directive to generate the linkbuf for the softlink you create, or use one of the other snap* directives. The older snapshot directive will continue to work as expected and in V3 it will also record the snapshot transaction id in file system meta-data. You may also want to make use of the new note tag for the meta-data.

WARNING! If you used to remove snapshot softlinks with rm you should probably start using the snaprm directive instead to also remove the related meta-data. The pruning code scans the meta-data so just removing the softlink is not sufficient.

This upgrade changes undo/flush, giving faster sync. It is possible to upgrade a V3 file system to V4 in place. This upgrade reformats the UNDO/REDO FIFO (typically 1GB), so upgrade might take a minute or two depending.

Version 4 allows the UNDO/REDO FIFO to be flushed without also having to flush the volume header, removing 2 of the 4 disk syncs typically required for an () and removing 1 of the 2 disk syncs typically required for a flush sequence. Version 4 also implements the REDO log (see FSYNC FLUSH MODES below) which is capable of fsync()ing with either one disk flush or zero disk flushes.

This upgrade brings in deduplication support. It is possible to upgrade a V4 file system to V5 in place. Technically it makes the layer2 bytes_free field a signed value instead of unsigned, allowing it to go negative. A version 5 filesystem is required for dedup operation.

It is possible to upgrade a V5 file system to V6 in place.

It is possible to upgrade a V6 file system to V7 in place. After upgrading any new files will use the new, faster CRC. You can convert all existing files to use the new CRC by issuing a full reblock via 'hammer reblock <fs>'. You only have to do this once.

HAMMER implements five different fsync flush modes via the vfs.hammer.fsync_mode sysctl, for HAMMER version 4+ file systems.

As of DragonFly 2.6 fsync mode 3 is set by default. REDO operation and recovery is enabled by default.

mode 0
Full synchronous fsync semantics without REDO.

HAMMER will not generate REDOs. A () will completely sync the data and meta-data and double-flush the FIFO, including issuing two disk synchronization commands. The data is guaranteed to be on the media as of when fsync() returns. Needless to say, this is slow.

mode 1
Relaxed asynchronous fsync semantics without REDO.

This mode works the same as mode 0 except the last disk synchronization command is not issued. It is faster than mode 0 but not even remotely close to the speed you get with mode 2 or mode 3.

Note that there is no chance of meta-data corruption when using this mode, it simply means that the data you wrote and then ()'d might not have made it to the media if the storage system crashes at a bad time.

mode 2
Full synchronous fsync semantics using REDO. NOTE: If not running a HAMMER version 4 filesystem or later mode 0 is silently used.

HAMMER will generate REDOs in the UNDO/REDO FIFO based on a heuristic. If this is sufficient to satisfy the () operation the blocks will be written out and HAMMER will wait for the I/Os to complete, and then followup with a disk sync command to guarantee the data is on the media before returning. This is slower than mode 3 and can result in significant disk or SSDs overheads, though not as bad as mode 0 or mode 1.

mode 3
Relaxed asynchronous fsync semantics using REDO. NOTE: If not running a HAMMER version 4 filesystem or later mode 1 is silently used.

HAMMER will generate REDOs in the UNDO/REDO FIFO based on a heuristic. If this is sufficient to satisfy the () operation the blocks will be written out and HAMMER will wait for the I/Os to complete, but will issue a disk synchronization command.

Note that there is no chance of meta-data corruption when using this mode, it simply means that the data you wrote and then ()'d might not have made it to the media if the storage system crashes at a bad time.

This mode is the fastest production fsyncing mode available. This mode is equivalent to how the UFS fsync in the BSDs operates.

mode 4
fsync is ignored.

Calls to () will be ignored. This mode is primarily designed for testing and should not be used on a production system.

You restore a snapshot by copying it over to live, but there is a caveat. The mtime and atime fields for files accessed via a snapshot is locked to the ctime in order to keep the snapshot consistent, because neither mtime nor atime changes roll any history.

In order to avoid unnecessary copying it is recommended that you use cpdup -VV -v when doing the copyback. Also make sure you traverse the snapshot softlink by appending a ".", as in "<snapshotpath>/.", and you match up the directory properly.

A PFS can be restored from a mirror with mirror-copy. config data must be copied separately. At last the PFS can be upgraded to master using pfs-upgrade.

It is not possible to restore the root PFS by using mirroring, as the root PFS is always a master PFS. A normal copy (e.g. using cpdup(1)) must be done, ignoring history. If history is important, old root PFS can me restored to a new PFS, and important directories/files can be null mounted to the new PFS.

The following environment variables affect the execution of hammer:

The editor program specified in the variable EDITOR will be invoked instead of the default editor, which is vi(1).
The command specified in the variable HAMMER_RSH will be used to initiate remote operations for the mirror-copy and mirror-stream commands instead of the default command, which is ssh(1). The program will be invoked via execvp(3) using a typical rsh(1) (net/bsdrcmds) style -l user host <remote-command> command line.
Same effect as EDITOR variable.

<pfs>/snapshots
default per PFS snapshots directory (HAMMER VERSION 2-)
/var/hammer/<pfs>
default per PFS snapshots directory (not root) (HAMMER VERSION 3+)
/var/hammer/root
default snapshots directory for root directory (HAMMER VERSION 3+)
<snapshots>/config
per PFS hammer cleanup configuration file (HAMMER VERSION 2-)
<fs>/var/slaves/<name>
recommended slave PFS snapshots directory (HAMMER VERSION 2-)
<fs>/pfs
recommended PFS directory

The hammer utility exits 0 on success, and >0 if an error occurs.

ssh(1), undo(1), HAMMER(5), periodic.conf(5), loader(8), mount_hammer(8), mount_null(8), newfs_hammer(8), swapcache(8), sysctl(8)

The hammer utility first appeared in DragonFly 1.11.

Matthew Dillon <dillon@backplane.com>

December 31, 2017 DragonFly-5.6.1