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GMIRROR(8)		FreeBSD	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 automati-
     cally.  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 sec-
     tor) 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, be-
		 cause a component's priority is based on its position (start-
		 ing 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 big-
					  ger than or equal to slice size on N
					  pieces, where	N is the number	of ac-
					  tive components.

		 -F	     Do	not synchronize	after a	power failure or sys-
			     tem crash.	 Assumes device	is in consistent

		 -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 compo-
			     nents.  Defaults to 4096 bytes.

     clear	 Clear metadata	on the given providers.

     create	 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	 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 sys-
			      tem crash.

		 -F	      Do not synchronize after a power failure or sys-
			      tem crash.  Assumes device is in consistent

		 -h	      Hardcode providers' names	in metadata.

		 -n	      Turn off autosynchronization of stale compo-

		 -p priority  Specifies	priority for the given component prov.

		 -s slice     Specifies	slice size for split balance algo-

     rebuild	 Rebuild the given mirror components forcibly.	If autosyn-
		 chronization 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

     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

		 -p priority  Specifies	priority of the	given component(s).

     remove	 Remove	the given component(s) from the	mirror and clear meta-
		 data on it.

     activate	 Activate the given component(s), which	were marked as inac-
		 tive 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 compo-

		 Additional options include:

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

     forget	 Forget	about components which are not connected.  This	com-
		 mand is useful	when a disk has	failed and cannot be recon-
		 nected, preventing the	remove command from being used to re-
		 move 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 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

     The following sysctl(8) variables can be used to configure	behavior for
     all mirrors.

	     Control the verbosity of kernel logging related to	mirrors.  A
	     value larger than 0 will enable debug logging.

	     The amount	of time, in seconds, to	wait for all copies of a mir-
	     ror to appear before starting the mirror.	Disks that appear af-
	     ter the mirror has	been started are not automatically added to
	     the mirror.

	     The amount	of time, in seconds, which must	elapse after the last
	     write to a	mirror before that mirror is marked clean.  Clean mir-
	     rors do not need to be synchronized after a power failure or sys-
	     tem crash.	 A small value may result in frequent overwrites of
	     the disks'	metadata sectors, and thus may reduce the longevity of
	     the disks.

	     Determine whether a disk is automatically removed from its	mirror
	     when an I/O request to that disk fails.

	     The number	of parallel I/O	requests used while synchronizing a
	     mirror.  This parameter may only be configured as a
	     loader.conf(5) tunable.

	     The period, in seconds, at	which a	synchronizing mirror's meta-
	     data is updated.  Periodic	updates	are used to record a synchro-
	     nization's	progress so that an interrupted	synchronization	may be
	     resumed starting at the recorded offset, rather than at the be-
	     ginning.  A smaller value results in more accurate	progress
	     tracking, but also	increases the number of	non-sequential writes
	     to	the disk being synchronized.  If the sysctl value is 0,	no up-
	     dates are performed until the synchronization is complete.

     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 compo-
     nent and gmirror always chooses the component with	the highest priority.
     Reading a dump from the mirror on boot will only work if the prefer bal-
     ance algorithm is used (that way gmirror will read	only from the compo-
     nent with the highest priority).  If you use a different balance algo-
     rithm, 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 compo-
     nent 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 pre-
     fer balance algorithm will	read from the next one,	thus will find nothing

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

     The gmirror utility appeared in FreeBSD 5.3.

     Pawel Jakub Dawidek <>

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

     There should be a section with an implementation description.

FreeBSD	13.0		       November	30, 2017		  FreeBSD 13.0


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