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+.\" Copyright (c) 2005 Pawel Jakub Dawidek <pjd@FreeBSD.org>
+.\" Copyright (c) 2005 Ivan Voras <ivoras@FreeBSD.org>
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\" $FreeBSD$
+.\"
+.Dd July 8, 2006
+.Dt GVIRSTOR 8
+.Os
+.Sh NAME
+.Nm gvirstor
+.Nd "provides virtual data storage geom"
+.Sh SYNOPSIS
+.Nm
+.Cm label
+.Op Fl hv
+.Op Fl s Ar virsize
+.Op Fl m Ar chunksize
+.Ar name
+.Ar prov ...
+.Nm
+.Cm stop
+.Op Fl fv
+.Ar name ...
+.Nm
+.Cm add
+.Op Fl vh
+.Ar name prov ...
+.Nm
+.Cm remove
+.Op Fl v
+.Ar name prov ...
+.Nm
+.Cm clear
+.Op Fl v
+.Ar prov ...
+.Nm
+.Cm dump
+.Ar prov ...
+.Nm
+.Cm list
+.Nm
+.Cm status
+.Nm
+.Cm load
+.Nm
+.Cm unload
+.Sh DESCRIPTION
+The
+.Nm
+utility is used for setting up a storage device of arbitrary large size (for example,
+several TB), consisting of an arbitrary number of physical storage devices with
+total size <= the virtual size. Data for the virtual devices will be allocated from
+physical devices on demand. In short, this is the virtual storage functionality.
+The first argument to
+.Nm
+indicates an action to be performed:
+.Bl -tag -width ".Cm destroy"
+.It Cm label
+Set up a virtual device from the given components with the specified
+.Ar name .
+Metadata are stored in the last sector of every component.
+Argument
+.Ar virsize
+is the size of new virtual device, with default being 2 TiB (2097152 MiB).
+Argument
+.Ar chunksize
+is the chunk size, with default being 4 MiB (4096 KiB).
+The default is thus "-s 2097152 -m 4096".
+.It Cm stop
+Turn off an existing virtual device by its
+.Ar name .
+This command does not touch on-disk metadata.
+As with other GEOM classes, stopped geoms cannot be started manually.
+.It Cm add
+Adds new components to existing virtual device by its
+.Ar name .
+The specified virstor device must exist and be active (i.e.
+module loaded, device present in /dev).
+.It Cm remove
+Removes components from existing virtual device by its
+.Ar name .
+Only unallocated providers can be removed.
+.It Cm clear
+Clear metadata on the given providers.
+.It Cm dump
+Dump metadata stored on the given providers.
+.It Cm list
+See
+.Xr geom 8 .
+.It Cm status
+See
+.Xr geom 8 .
+.It Cm load
+See
+.Xr geom 8 .
+.It Cm unload
+See
+.Xr geom 8 .
+.El
+.Pp
+Additional options:
+.Bl -tag -width ".Fl f"
+.It Fl f
+Force the removal of the specified virtual device.
+.It Fl h
+Hardcode providers' names in metadata.
+.It Fl v
+Be more verbose.
+.El
+.Sh EXIT STATUS
+Exit status is 0 on success, and 1 if the command fails.
+.Sh 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.
+.Bd -literal -offset indent
+gvirstor label -v mydata /dev/ad4 /dev/ad6
+newfs /dev/virstor/mydata
+.Ed
+.Pp
+From now on, the virtual device will be available via the
+.Pa /dev/virstor/mydata
+device entry.
+To add a new physical device / provider to an active virstor device:
+.Bd -literal -offset indent
+gvirstor add mydata ad8
+.Ed
+.Pp
+This will add physical storage (from ad8) to 
+.Pa /dev/virstor/mydata
+device.
+To see device status information (including how much physical storage 
+is still available for the virtual device), use:
+.Bd -literal -offset indent
+gvirstor list
+.Ed
+.Pp
+All standard
+.Xr geom 8
+subcommands (e.g. "status", "help") are also supported.
+.Sh SYSCTLs
+.Nm
+has several 
+.Xr sysctl 8
+tunable variables.
+.Bd -literal -offset indent
+.Pa int kern.geom.virstor.debug
+.Ed
+.Pp
+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 supressed. Default value is 5 and it's not
+recommented to set this tunable to less than 2, because level 1 messages 
+are error events, and level 2 messages are system warnings.
+.Bd -literal -offset indent
+.Pa int kern.geom.virstor.chunk_watermark
+.Ed
+.Pp
+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).
+.Bd -literal -offset indent
+.Pa int kern.geom.virstor.component_watermark
+.Ed
+.Pp
+Value in this sysctl sets warning watermark level for component usage.
+The warning is issed when there are less than this many unallocated
+components (default is 1).
+.Pp
+All these sysctls are also available as
+.Xr loader 8
+tunables.
+.Sh LOG MESSAGES
+.Nm
+kernel module issues log messages with prefixes in standardised format,
+which is useful for log message filtering and dispatching. Each message
+line begins with
+.Bd -literal -offset indent
+.Pa GEOM_VIRSTOR[%d]: 
+.Ed
+.Pp
+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).
+.Sh SEE ALSO
+.Xr geom 4 ,
+.Xr geom 8 ,
+.Xr newfs 8 ,
+.Xr fstab 5 ,
+.Xr glabel 8
+.Sh HISTORY
+The
+.Nm
+utility appeared in
+.Fx 7.0 .
+.Sh 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's recommended to run them when the system is quiescent.
+.Sh AUTHOR
+.An Ivan Voras Aq ivoras@FreeBSD.org 
+Sponsored by Google Summer of Code 2006