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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
     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:

     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

                                       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

                                       split            Split read requests,
                                                        which are bigger than
                                                        or equal to slice size
                                                        on N pieces, where N
                                                        is the number of
                                                        active components.

                     -h                Hardcode providers' names in metadata.

                     -n                Turn off autosynchronization of stale

                     -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 component.

     clear           Clear metadata on the given providers.

     configure       Configure the given device.

                     Additional options include:

                     -a                Turn on autosynchronization of stale

                     -b balance        Specifies balance algorithm to use.

                     -d                Do not hardcode providers' names in

                     -h                Hardcode providers' names in metadata.

                     -n                Turn off autosynchronization of stale

                     -s slice          Specifies slice size for split balance

     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

     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.

     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

     geom(4), geom(8), mount(8), newfs(8), umount(8), vinum(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 11.0-PRERELEASE          July 9, 2004          FreeBSD 11.0-PRERELEASE


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