NAME
WAPBL —
Write Ahead Physical Block Logging file
system journaling
SYNOPSIS
options WAPBL
options WAPBL_DEBUG
DESCRIPTION
TheWAPBL driver provides meta-data journaling for file
systems. In particular, it is used with the fast file system (FFS) to provide
rapid file system consistency checking after a system outage. It also provides
better general-use performance over regular FFS.
WAPBL currently maintains its journal in one of two locations:
- - After the file system
- The journal is placed in the same partition as the file system, but between the file system and the end of the partition.
- - Within the file system
- The journal is allocated as a special contiguous file within the file system. The journal file is not visible via normal file system access.
A new journal is created automatically when a file system is
mounted via mount(8) with the -o
log option. If no journal size has been specified with
tunefs(8), then the size of the journal will be based on 1MB of
journal per 1GB of file system, to a maximum journal size of 64MB.
If there is adequate space between the end of the file system and the end of the partition, then unless the journal size has been specified with tunefs(8) then the journal will be created after the file system. To obtain space between the file system and the end of the partition the size of the partition can be adjusted using disklabel(8). Care must be taken not to damage existing data on existing partitions, but this method will work well if, for example, a swap partition can be shrunk in order to accommodate the journal after the file system on a partition before the swap partition.
For a new file system,
newfs -s -64m wd0a
can be used to leave space for a 64MB journal at the end of /dev/wd0a.
To specify the size of the journal within the file system tunefs(8) can be used as follows:
tunefs -l 64m wd0a
to indicate that a journal of size 64MB on the file system on /dev/wd0a should be created the next time that file system is mounted. This must be done before the file system is mounted with the “-o log” option. For existing file systems and general use, however, simply using
mount -o log /dev/wd0a /mnt
will be sufficient to create an appropriate journal within the file system. Running
tunefs -l 0 wd0a
will schedule the log for removal on the next read-write mount, and running
tunefs -l 0 wd0a
followed by
mount -o log /dev/wd0a /mnt
will remove the log and then re-create it with the default size. This method can also be used to grow or shrink the size of the journal by first scheduling the log for removal, then mounting read-write, but with logging disabled (so no new log will be created), then unmounting again, setting the desired log size and finally re-mounting with logging enabled.
With the journal, fsck(8) is no longer required at system boot. If the system has been shutdown in an unclean fashion then the journal will be replayed when the file system is mounted. fsck(8) can still be used to force a consistency check of the file system should that be desired.
For kernel developers, the compile time option
WAPBL_DEBUG turns on debugging.
SEE ALSO
HISTORY
WAPBL was originally written by
Darrin B. Jewell while at Wasabi Systems Inc. Wasabi
Systems contributed the code to NetBSD, and it was
integrated by Simon Burge, Antti
Kantee, Andy Doran, and Greg
Oster.
WAPBL first appeared in
NetBSD 5.0.
CAVEATS
Older releases of the system, and other systems that support the
UFS format should only access
WAPBL file systems in read-only mode. Additionally,
the fsck(8) command from such systems should not be run against
WAPBL file systems. Failure to observe these
guidelines may damage the file system.
WAPBL requires the super block to be in
the UFS2 format. The super block format can be checked using the
-s option with
dumpfs(8), and older FFSv1 file systems will need to be updated to
the newer super block layout with the -c option to
fsck_ffs(8).
fsync(2) causes all outstanding metadata transactions to be committed to disk, introducing additional latency. This can have an impact on database software and other software that calls fsync(2) often.
In-file system log allocation should be done on a relatively quiet file system. The error path for log allocation failures could result in a “dangling inode” issue, requiring an fsck(8) to fix.