The
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
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
indicates an action to be performed:
label
Create a mirror.
The order of components is important, because a component's priority is based on its position
(starting from 0).
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:
-b balance
Specifies balance algorithm to use, one of:
load
Read from the component with the lowest load.
prefer
Read from the component with the biggest priority.
round-robin
Use round-robin algorithm when choosing component to read.
split
Split read requests, which are bigger than or equal to slice size on N pieces,
where N is the number of active components.
This is the default balance algorithm.
-F
Do not synchronize after a power failure or system crash.
Assumes device is in consistent state.
-h
Hardcode providers' names in metadata.
-n
Turn off autosynchronization of stale components.
-s 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
Clear metadata on the given providers.
configure
Configure the given device.
Additional options include:
-a
Turn on autosynchronization of stale components.
-b balance
Specifies balance algorithm to use.
-d
Do not hardcode providers' names in metadata.
-f
Synchronize device after a power failure or system crash.
-F
Do not synchronize after a power failure or system crash.
Assumes device is in consistent state.
-h
Hardcode providers' names in metadata.
-n
Turn off autosynchronization of stale components.
-s slice
Specifies slice size for
split
balance algorithm.
rebuild
Rebuild the given mirror components forcibly.
If autosynchronization was not turned off for the given device, this command
should be unnecessary.
insert
Add the given component(s) to the existing mirror.
Additional options include:
-h
Hardcode providers' names in metadata.
-i
Mark component(s) as inactive immediately after insertion.
-p priority
Specifies priority of the given component(s).
remove
Remove the given component(s) from the mirror and clear metadata on it.
activate
Activate the given component(s), which were marked as inactive before.
deactivate
Mark the given component(s) as inactive, so it will not be automatically
connected to the mirror.
forget
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.
Exit status is 0 on success, and 1 if the command fails.
EXAMPLES
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
NOTES
Doing kernel dumps to
providers.
This is possible, but some conditions have to be met.
First of all, a kernel dump will go only to one component and
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
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.
