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

NAME

     gmirror -- control utility for mirrored devices

SYNOPSIS

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

DESCRIPTION

     The gmirror utility is used for mirror (RAID1) configurations.  After a mirror's creation, all components are detected and configured auto-
     matically.  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:

     label	 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:

		 -b balance  Specifies balance algorithm to use, one of:

			     load	  Read from the component with the lowest load.  This is the default balance algorithm.

			     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.

		 -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.

		 -p priority  Specifies priority for the given component prov.

		 -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.

     resize	 Change the size of the given mirror.

		 Additional options include:

		 -s 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.

     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.

     destroy	 Stop the given mirror and clear metadata on all its components.

		 Additional options include:

		 -f  Stop the given mirror even if it is opened.

     forget	 Forget about components which are not connected.  This command is useful when a disk has failed and cannot be reconnected, pre-
		 venting the remove command from being used to remove it.

     stop	 Stop the given mirror.

		 Additional options include:

		 -f  Stop the given mirror even if it is opened.

     dump	 Dump metadata stored on the given providers.

     list	 See geom(8).

     status	 See geom(8).

     load	 See geom(8).

     unload	 See geom(8).

     Additional options include:

     -v  Be more verbose.

EXIT STATUS

     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 sys-
     tem, 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 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 bal-
     ance 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.

SEE ALSO

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

HISTORY

     The gmirror utility appeared in FreeBSD 5.3.

AUTHORS

     Pawel Jakub Dawidek <pjd@FreeBSD.org>

BUGS

     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.

BSD
								 December 27, 2013							       BSD