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

NAME
     gvirstor - control utility for virtual data storage devices

SYNOPSIS
     gvirstor label [-hv] [-s virsize] [-m chunksize] name prov ...
     gvirstor stop [-fv] name ...
     gvirstor destroy [-fv] name ...
     gvirstor add [-vh] name prov ...
     gvirstor remove [-v] name prov ...
     gvirstor clear [-v] prov ...
     gvirstor dump prov ...
     gvirstor list
     gvirstor status
     gvirstor load
     gvirstor unload

DESCRIPTION
     The gvirstor utility is used for setting up a virtual storage device of
     arbitrary large size (for example, several TB), consisting of an
     arbitrary number of physical storage devices with the total size which is
     equal to or smaller than the virtual size.  Data for the virtual devices
     will be allocated from physical devices on demand.  The idea behind
     gvirstor is similar to the concept of Virtual Memory in operating
     systems, effectively allowing users to overcommit on storage (free file
     system space).  The concept is also known as "thin provisioning" in
     virtualization environments, only here it is implemented on the level of
     physical storage devices.

     The first argument to gvirstor indicates an action to be performed:

     label       Set up a virtual device from the given components with the
                 specified name.  Metadata is stored in the last sector of
                 every component.  Argument -s virsize is the size of new
                 virtual device, with default being set to 2 TiB (2097152
                 MiB).  Argument -m chunksize is the chunk size, with default
                 being set to 4 MiB (4096 KiB).  The default arguments are
                 thus "-s 2097152 -m 4096".

     stop        Turn off an existing virtual device with the given name.
                 This command does not touch on-disk metadata.  As with other
                 GEOM classes, stopped geoms cannot be started manually.

     destroy     Same as stop.

     add         Adds new components to existing virtual device with the given
                 name.  The specified virstor device must exist and be active
                 (i.e. module loaded, device present in /dev).  This action
                 can be safely performed while the virstor device is in use
                 ("hot" operation).

     remove      Removes components from existing virtual device with the
                 given name.  Only unallocated providers can be removed.

     clear       Clear metadata on the given providers.

     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:

     -f     Force the removal of the specified virtual device.

     -h     Hardcode providers' names in metadata.

     -v     Be more verbose.

EXAMPLES
     The following example shows how to create a virtual device of default
     size (2 TiB), of default chunk (extent) size (4 MiB), with two physical
     devices for backing storage.

           gvirstor label -v mydata /dev/ada4 /dev/ada6
           newfs /dev/virstor/mydata

     From now on, the virtual device will be available via the
     /dev/virstor/mydata device entry.  To add a new physical device /
     component to an active virstor device:

           gvirstor add mydata ada8

     This will add physical storage of ada8 to /dev/virstor/mydata device.

     To see the device status information (including how much physical storage
     is still available for the virtual device), use:

           gvirstor list

     All standard geom(8) subcommands (e.g. status, help) are also supported.

SYSCTL VARIABLES
     gvirstor has several sysctl(8) tunable variables.

           int kern.geom.virstor.debug

     This sysctl controls verbosity of the kernel module, in the range 1 to
     15.  Messages that are marked with higher verbosity levels than this are
     suppressed.  Default value is 5 and it is not recommended to set this
     tunable to less than 2, because level 1 messages are error events, and
     level 2 messages are system warnings.

           int kern.geom.virstor.chunk_watermark

     Value in this sysctl sets warning watermark level for physical chunk
     usage on a single component.  The warning is issued when a virstor
     component has less than this many free chunks (default 100).

           int kern.geom.virstor.component_watermark

     Value in this sysctl sets warning watermark level for component usage.
     The warning is issued when there are less than this many unallocated
     components (default is 1).

     All these sysctls are also available as loader(8) tunables.

DIAGNOSTICS
     The gvirstor utility exits 0 on success, and >0 if an error occurs.

     gvirstor kernel module issues log messages with prefixes in standardized
     format, which is useful for log message filtering and dispatching.  Each
     message line begins with

           GEOM_VIRSTOR[%d]:

     The number (%d) is message verbosity / importance level, in the range 1
     to 15.  If a message filtering, dispatching or operator alert system is
     used, it is recommended that messages with levels 1 and 2 be taken
     seriously (for example, to catch out-of-space conditions as set by
     watermark) sysctls.

SEE ALSO
     geom(4), fstab(5), geom(8), glabel(8), newfs(8)

HISTORY
     The gvirstor utility first appeared in FreeBSD 7.0.

BUGS
     Commands add and remove contain unavoidable critical sections which may
     make the virstor device unusable if a power failure (or other disruptive
     event) happens during their execution.  It is recommended to run them
     when the system is quiescent.

ASSUMPTIONS AND INTERACTION WITH FILE SYSTEMS
     There are several assumptions that gvirstor has in its operation: that
     the size of the virtual storage device will not change once it is set,
     and that the sizes of individual physical storage components will always
     remain constant during their existence.  For alternative ways to
     implement virtual or resizable file systems see zfs(1M), gconcat(8) and
     growfs(8).

     Note that gvirstor has nontrivial interaction with file systems which
     initialize a large number of on-disk structures during newfs.  If such
     file systems attempt to spread their structures across the drive media
     (like UFS/UFS2 does), their efforts will be effectively foiled by
     sequential allocation of chunks in gvirstor and all their structures will
     be physically allocated at the start of the first virstor component.
     This could have a significant impact on file system performance (which
     can in some rare cases be even positive).

AUTHOR
     Ivan Voras <ivoras@FreeBSD.org>

     Sponsored by Google Summer of Code 2006.

FreeBSD 11.0-PRERELEASE         October 1, 2013        FreeBSD 11.0-PRERELEASE

NAME | SYNOPSIS | DESCRIPTION | EXAMPLES | SYSCTL VARIABLES | DIAGNOSTICS | SEE ALSO | HISTORY | BUGS | ASSUMPTIONS AND INTERACTION WITH FILE SYSTEMS | AUTHOR

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