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.
AUTHORS
Ivan Voras <ivoras@FreeBSD.org>
Sponsored by Google Summer of Code 2006.
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).
FreeBSD 13.1-RELEASE-p6 October 1, 2013 FreeBSD 13.1-RELEASE-p6
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