gvirstor man page on FreeBSD

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GVIRSTOR(8)		  BSD 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 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 arbi‐
     trary 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 sys‐
     tems, effectively allowing users to overcommit on storage (free file
     system space).  The first argument to gvirstor indicates an action to be
     performed:

     label   Set up a virtual device from the given components with the speci‐
	     fied name.	 Metadata is stored in the last sector of every compo‐
	     nent.  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.

     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/ad4 /dev/ad6
	   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 / compo‐
     nent to an active virstor device:

	   gvirstor add mydata ad8

     This will add physical storage of ad8 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 compo‐
     nent 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 com‐
     ponents (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 seri‐
     ously (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 imple‐
     ment 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 sequen‐
     tial 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.

BSD			       December 17, 2008			   BSD
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