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 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.

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 / component 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 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

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.