Several significant updates:
* Better wording of sections dealing with physical storage * A new section on assumptions gvirstor has on its consumer devices (components) and its interaction with file systems * Improved markup (by hrs@) Reviewed by: hrs Approved by: gnn (mentor)
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.\" Copyright (c) 2005 Pawel Jakub Dawidek <pjd@FreeBSD.org>
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.\" Copyright (c) 2005 Ivan Voras <ivoras@FreeBSD.org>
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.\" Copyright (c) 2006-2008 Ivan Voras <ivoras@FreeBSD.org>
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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@ -25,7 +24,7 @@
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.\"
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.\" $FreeBSD$
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.\"
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.Dd July 8, 2006
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.Dd December 17, 2008
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.Dt GVIRSTOR 8
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.Os
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.Sh NAME
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@ -69,10 +68,17 @@
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.Sh DESCRIPTION
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The
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.Nm
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utility is used for setting up a storage device of arbitrary large size (for example,
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several TB), consisting of an arbitrary number of physical storage devices with
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total size <= the virtual size. Data for the virtual devices will be allocated from
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physical devices on demand. In short, this is the virtual storage functionality.
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utility is used for setting up a virtual storage device of arbitrary
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large size
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.Pq for example, several TB ,
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consisting of an arbitrary number of physical storage devices with the
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total size which is equal to or smaller than the virtual size. Data
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for the virtual devices will be allocated from physical devices on
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demand. The idea behind
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.Nm
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is similar to the concept of Virtual Memory in operating systems,
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effectively allowing users to overcommit on storage
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.Pq free file system space .
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The first argument to
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.Nm
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indicates an action to be performed:
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@ -82,12 +88,15 @@ Set up a virtual device from the given components with the specified
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.Ar name .
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Metadata are stored in the last sector of every component.
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Argument
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.Ar virsize
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is the size of new virtual device, with default being 2 TiB (2097152 MiB).
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.Fl s Ar virsize
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is the size of new virtual device, with default being 2 TiB
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.Pq 2097152 MiB .
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Argument
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.Ar chunksize
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is the chunk size, with default being 4 MiB (4096 KiB).
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The default is thus "-s 2097152 -m 4096".
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.Fl m Ar chunksize
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is the chunk size, with default being 4 MiB
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.Pq 4096 KiB .
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The default is thus
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.Qq Fl s Ar 2097152 Fl m Ar 4096 .
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.It Cm stop
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Turn off an existing virtual device by its
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.Ar name .
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@ -96,8 +105,10 @@ As with other GEOM classes, stopped geoms cannot be started manually.
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.It Cm add
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Adds new components to existing virtual device by its
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.Ar name .
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The specified virstor device must exist and be active (i.e.
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module loaded, device present in /dev).
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The specified virstor device must exist and be active
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.Pq i.e. module loaded, device present in Pa /dev .
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This action can be safely performed while the virstor device is in use
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.Pq Qo hot Qc operation
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.It Cm remove
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Removes components from existing virtual device by its
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.Ar name .
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@ -130,82 +141,97 @@ Hardcode providers' names in metadata.
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Be more verbose.
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.El
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.Sh EXIT STATUS
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Exit status is 0 on success, and 1 if the command fails.
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The
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.Nm
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utility exits 0 on success, and 1 if an error occurs.
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.Sh EXAMPLES
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The following example shows how to create a virtual device of default size
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(2 TiB), of default chunk (extent) size (4 MiB), with two physical devices for
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backing storage.
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.Pq 2 TiB ,
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of default chunk
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.Pq extent
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size
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.Pq 4 MiB ,
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with two physical devices for backing storage.
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.Bd -literal -offset indent
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gvirstor label -v mydata /dev/ad4 /dev/ad6
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newfs /dev/virstor/mydata
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.No gvirstor label -v Ar mydata Ar /dev/ad4 Ar /dev/ad6
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.No newfs Ar /dev/virstor/mydata
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.Ed
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.Pp
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From now on, the virtual device will be available via the
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.Pa /dev/virstor/mydata
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device entry.
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To add a new physical device / provider to an active virstor device:
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To add a new physical device / component to an active virstor device:
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.Bd -literal -offset indent
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gvirstor add mydata ad8
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.No gvirstor add Ar mydata Ar ad8
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.Ed
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.Pp
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This will add physical storage (from ad8) to
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This will add physical storage
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.Ar ad8
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to
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.Pa /dev/virstor/mydata
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device.
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To see device status information (including how much physical storage
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is still available for the virtual device), use:
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To see device status information
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.Pq including how much physical storage is still available for the virtual device ,
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use:
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.Bd -literal -offset indent
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gvirstor list
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.Ed
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.Pp
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All standard
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.Xr geom 8
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subcommands (e.g. "status", "help") are also supported.
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.Sh SYSCTLs
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subcommands
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.Pq e.g. Cm status , Cm help
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are also supported.
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.Sh SYSCTL VARIABLES
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.Nm
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has several
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has several
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.Xr sysctl 8
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tunable variables.
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.Bd -literal -offset indent
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.Pa int kern.geom.virstor.debug
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.Va int kern.geom.virstor.debug
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.Ed
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.Pp
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This sysctl controls verbosity of the kernel module, in the range
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1 to 15. Messages that are marked with higher verbosity levels than
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this are supressed. Default value is 5 and it's not
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recommented to set this tunable to less than 2, because level 1 messages
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1 to 15. Messages that are marked with higher verbosity levels than
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this are suppressed. Default value is 5 and it's not
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recommended to set this tunable to less than 2, because level 1 messages
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are error events, and level 2 messages are system warnings.
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.Bd -literal -offset indent
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.Pa int kern.geom.virstor.chunk_watermark
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.Va int kern.geom.virstor.chunk_watermark
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.Ed
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.Pp
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Value in this sysctl sets warning watermark level for physical chunk usage
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on a single component. The warning is issued when a virstor component
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has less than this many free chunks (default 100).
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has less than this many free chunks
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.Pq default 100 .
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.Bd -literal -offset indent
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.Pa int kern.geom.virstor.component_watermark
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.Va int kern.geom.virstor.component_watermark
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.Ed
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.Pp
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Value in this sysctl sets warning watermark level for component usage.
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The warning is issed when there are less than this many unallocated
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components (default is 1).
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The warning is issued when there are less than this many unallocated
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components
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.Pq default is 1 .
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.Pp
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All these sysctls are also available as
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.Xr loader 8
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tunables.
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.Sh LOG MESSAGES
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.Sh DIAGNOSTICS
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.Nm
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kernel module issues log messages with prefixes in standardised format,
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kernel module issues log messages with prefixes in standardized format,
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which is useful for log message filtering and dispatching. Each message
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line begins with
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.Bd -literal -offset indent
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.Pa GEOM_VIRSTOR[%d]:
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.Li GEOM_VIRSTOR[%d]:
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.Ed
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.Pp
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The number (%d) is message verbosity / importance level, in the range
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1 to 15. If a message filtering, dispatching or operator alert system is
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used, it is recommended that messages with levels 1 and 2 be taken
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seriously (for example, to catch out-of-space conditions as set by
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watermark sysctls).
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The number
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.Pq %d
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is message verbosity / importance level, in the range 1 to 15. If a
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message filtering, dispatching or operator alert system is used, it is
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recommended that messages with levels 1 and 2 be taken seriously
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.Pq for example, to catch out-of-space conditions as set by watermark
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sysctls .
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.Sh SEE ALSO
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.Xr geom 4 ,
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.Xr geom 8 ,
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@ -218,10 +244,40 @@ The
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utility appeared in
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.Fx 7.0 .
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.Sh BUGS
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Commands "add" and "remove" contain unavoidable critical sections
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which may make the virstor device unusable if a power failure (or
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other disruptive event) happens during their execution.
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It's recommended to run them when the system is quiescent.
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Commands
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.Cm add
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and
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.Cm remove
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contain unavoidable critical sections which may make the virstor
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device unusable if a power failure
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.Pq or other disruptive event
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happens during their execution. It's recommended to run them when the
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system is quiescent.
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.Sh ASSUMPTIONS AND INTERACTION WITH FILE SYSTEMS
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There are several assumptions that
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.Nm
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has in its operation: that the size of the virtual storage device will not
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change once it's set, and that the sizes of individual physical storage
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components will always remain constant during their existence. For
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alternative ways to implement virtual or resizable file systems see
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.Xr zfs 1M ,
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.Xr gconcat 8 and
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.Xr growfs 8 .
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.Pp
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Note that
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.Nm
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has nontrivial interaction with file systems which initialize a large
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number of on-disk structures during newfs. If such file systems
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attempt to spread their structures across the drive media
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.Pq like UFS/UFS2 does ,
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their efforts will be effectively foiled by sequential allocation of
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chunks in
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.Nm
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and all their structures will be physically allocated at the start
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of the first virstor component. This could have a significant impac
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t on file system performance
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.Pq which can in some rare cases be even positive .
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.Sh AUTHOR
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.An Ivan Voras Aq ivoras@FreeBSD.org
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.An Ivan Voras Aq ivoras@FreeBSD.org
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.Pp
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Sponsored by Google Summer of Code 2006
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