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

     gmirror -- control utility for mirrored devices

     gmirror label [-hnv] [-b balance] [-s slice] name prov ...
     gmirror clear [-v] prov ...
     gmirror configure [-adhnv] [-b balance] [-s slice] name
     gmirror rebuild [-v] name prov ...
     gmirror insert [-hiv] [-p priority] name prov ...
     gmirror remove [-v] name prov ...
     gmirror activate [-v] name prov ...
     gmirror deactivate [-v] name prov ...
     gmirror forget [-v] name ...
     gmirror stop [-fv] name ...
     gmirror clear [-v] prov ...
     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,
                 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 big-
                                          ger than or equal to slice size on N
                                          pieces, where N is the number of
                                          active components.  This is the
                                          default balance algorithm.

                 -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.

     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.

                 -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 autosyn-
                 chronization 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

                 -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.

     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
                 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 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.

     This 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 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), mount(8), newfs(8), savecore(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.

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

FreeBSD 6.2                      March 8, 2006                     FreeBSD 6.2


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