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

gmirror — control utility for mirrored devices

gmirror label [-Fhnv] [-b balance] [-s slice] name prov ...

gmirror clear [-v] prov ...

gmirror create [-Fnv] [-b balance] [-s slice] name prov ...

gmirror configure [-adfFhnv] [-b balance] [-s slice] name

gmirror configure [-v] -p priority name prov

gmirror rebuild [-v] name prov ...

gmirror resize [-v] [-s size] name

gmirror insert [-hiv] [-p priority] name prov ...

gmirror remove [-v] name prov ...

gmirror activate [-v] name prov ...

gmirror deactivate [-v] name prov ...

gmirror destroy [-fv] name ...

gmirror forget [-v] name ...

gmirror stop [-fv] name ...

gmirror dump prov ...

gmirror list

gmirror status

gmirror load

gmirror unload

The gmirror utility is used for mirror (RAID1) configurations. After a mirror's creation, all components are detected and configured automatically. All operations like failure detection, stale component detection, rebuild of stale components, etc. are also done automatically. The gmirror utility uses on-disk metadata (stored in the provider's last sector) to store all needed information. Since the last sector is used for this purpose, it is possible to place a root file system on a mirror.

The first argument to gmirror indicates an action to be performed:

Create a mirror. The order of components is important, because a component's priority is based on its position (starting from 0 to 255). The component with the biggest priority is used by the prefer balance algorithm and is also used as a master component when resynchronization is needed, e.g. after a power failure when the device was open for writing.

Additional options include:

balance
Specifies balance algorithm to use, one of:
Read from the component with the lowest load. This is the default balance algorithm.
Read from the component with the biggest priority.
Use round-robin algorithm when choosing component to read.
Split read requests, which are bigger than or equal to slice size on N pieces, where N is the number of active components.
Do not synchronize after a power failure or system crash. Assumes device is in consistent state.
Hardcode providers' names in metadata.
Turn off autosynchronization of stale components.
slice
When using the split balance algorithm and an I/O READ request is bigger than or equal to this value, the I/O request will be split into N pieces, where N is the number of active components. Defaults to 4096 bytes.
Clear metadata on the given providers.
Similar to label, but creates mirror without storing on-disk metadata in last sector. This special "manual" operation mode assumes some external control to manage mirror detection after reboot, device hot-plug and other external events.
Configure the given device.

Additional options include:

Turn on autosynchronization of stale components.
balance
Specifies balance algorithm to use.
Do not hardcode providers' names in metadata.
Synchronize device after a power failure or system crash.
Do not synchronize after a power failure or system crash. Assumes device is in consistent state.
Hardcode providers' names in metadata.
Turn off autosynchronization of stale components.
priority
Specifies priority for the given component prov.
slice
Specifies slice size for split balance algorithm.
Rebuild the given mirror components forcibly. If autosynchronization was not turned off for the given device, this command should be unnecessary.
Change the size of the given mirror.

Additional options include:

size
New size of the mirror is expressed in logical block numbers. This option can be omitted, then it will be automatically calculated to maximum available size.
Add the given component(s) to the existing mirror.

Additional options include:

Hardcode providers' names in metadata.
Mark component(s) as inactive immediately after insertion.
priority
Specifies priority of the given component(s).
Remove the given component(s) from the mirror and clear metadata on it.
Activate the given component(s), which were marked as inactive before.
Mark the given component(s) as inactive, so it will not be automatically connected to the mirror.
Stop the given mirror and clear metadata on all its components.

Additional options include:

Stop the given mirror even if it is opened.
Forget about components which are not connected. This command is useful when a disk has failed and cannot be reconnected, preventing the remove command from being used to remove it.
Stop the given mirror.

Additional options include:

Stop the given mirror even if it is opened.
Dump metadata stored on the given providers.
See geom(8).
See geom(8).
See geom(8).
See geom(8).

Additional options include:

Be more verbose.

Exit status is 0 on success, and 1 if the command fails.

Use 3 disks to setup a mirror. Choose split balance algorithm, split only requests which are bigger than or equal to 2kB. Create file system, mount it, then unmount it and stop device:

gmirror label -v -b split -s 2048 data da0 da1 da2
newfs /dev/mirror/data
mount /dev/mirror/data /mnt
...
umount /mnt
gmirror stop data
gmirror unload

Create a mirror on disk with valid data (note that the last sector of the disk will be overwritten). Add another disk to this mirror, so it will be synchronized with existing disk:

gmirror label -v -b round-robin data da0
gmirror insert data da1

Create a mirror, but do not use automatic synchronization feature. Add another disk and rebuild it:

gmirror label -v -n -b load data da0 da1
gmirror insert data da2
gmirror rebuild data da2

One disk failed. Replace it with a brand new one:

gmirror forget data
gmirror insert data da1

Create a mirror, deactivate one component, do the backup and connect it again. It will not be resynchronized, if there is no need to do so (there were no writes in the meantime):

gmirror label data da0 da1
gmirror deactivate data da1
dd if=/dev/da1 of=/backup/data.img bs=1m
gmirror activate data da1

Doing kernel dumps to gmirror providers is possible, but some conditions have to be met. First of all, a kernel dump will go only to one component and gmirror always chooses the component with the highest priority. Reading a dump from the mirror on boot will only work if the prefer balance algorithm is used (that way gmirror will read only from the component with the highest priority). If you use a different balance algorithm, you should add:

gmirror configure -b prefer data

to the /etc/rc.early script and:

gmirror configure -b round-robin data

to the /etc/rc.local script. The decision which component to choose for dumping is made when dumpon(8) is called. If on the next boot a component with a higher priority will be available, the prefer algorithm will choose to read from it and savecore(8) will find nothing. If on the next boot a component with the highest priority will be synchronized, the prefer balance algorithm will read from the next one, thus will find nothing there.

geom(4), dumpon(8), geom(8), gvinum(8), mount(8), newfs(8), savecore(8), umount(8)

The gmirror utility appeared in FreeBSD 5.3.

Pawel Jakub Dawidek <pjd@FreeBSD.org>

There should be a way to change a component's priority inside a running mirror.

There should be a section with an implementation description.

Documentation for sysctls kern.geom.mirror.* is missing.

November 27, 2016 FreeBSD-11.1