FSSNAP_UFS(8) Maintenance Commands and Procedures FSSNAP_UFS(8)

NAME

fssnap_ufs - create a temporary snapshot of a UFS file system

SYNOPSIS

fssnap [-F ufs] [-V] -o backing-store=path,
[specific-options] /mount/point


fssnap -d [-F ufs] [-V] /mount/point | dev


fssnap -i [-F ufs] [-V] [-o specific-options] /mount/point | dev

DESCRIPTION

The fssnap command queries, creates, or deletes a temporary snapshot
of a UFS file system. A snapshot is a point-in-time image of a file
system that provides a stable and unchanging device interface for
backups.


When creating a file system snapshot, you must specify the file
system to be captured and the backing-store file. The backing-store
file(s) are where the snapshot subsystem saves old file system data
before it is overwritten. Beyond the first backing-store file, fssnap
automatically creates additional backing-store files on an as-needed
basis.


The number and size of the backing store files varies with the amount
of activity in the file system. The destination path must have enough
free space to hold the backing-store file(s). This location must be
different from the file system that is being captured in a snapshot.
The backing-store file(s) can reside on any type of file system,
including another UFS file system or an NFS-mounted file system.

OPTIONS

The following options are supported:

-d

Deletes the snapshot associated with the given file system.


-i

Displays the state of one or all UFS snapshots. If a mount-point
or device is not specified, a list of all snapshots on the system
is displayed. When a mount-point or device is specified,
detailed information is provided for the specified file system
snapshot by default.

Use the -o options with the -i option to specify what snapshot
information is displayed. Since this feature is provided
primarily for use in scripts and on the command line, no labels
are displayed for the data. Sizes are all in bytes, and the
output is not internationalized or localized. The information is
displayed on one line per option. Unrecognized options display a
single ? on the line. One line per option guarantees that there
are the same number of lines as options specified and there is a
one-to-one correspondence between an output line and an option.

The following -o options display specific information for a given
snapshot. See the EXAMPLES section for examples of how to use
these options.

snapnumber

Display the snapshot number.


blockdevname

Display the block device path.


rawdevname

Display the raw device path.


mountpoint

Display the mount point of the master file system.


state

Display the state of the snapshot device.


backing-store

Display the location of the first backing-store file for this
snapshot. If there are multiple backing-store files,
subsequent files have the same name as the first file, with
the suffixes .2, .3, and so forth.


backing-store-len

Display the sum of the sizes of the backing-store files.


maxsize

Display the maxsize value specified for the backing-store
file(s).


createtime

Display the time that the snapshot was created.


chunksize

Display the copy-on-write granularity.


-o specific-options

Without -d or -i, the default action is to create a snapshot.
Specify the following options when creating a snapshot. All of
these options are discretionary, except for the backing-store
file, which is required.

backing-store=path

Uses path in the creation of the backing-store file(s). path
must not reside on the file system that is being captured in
a snapshot and must not be the name of an existing file. If
path is a directory, then a backing-store file is created
within it using a name that is generated automatically. If
path is not a directory and does not already exist, then a
backing-store file with that name is created. If more than
one backing-store file is required, fssnap creates subsequent
files automatically. The second and subsequent files have the
same name as the first file, with suffixes of .2, .3, and so
forth.

This option can be abbreviated as bf=path or bs=path.


unlink

Unlinks the backing-store file after the snapshot is created.
This option specifies that the backing-store file does not
need to be removed manually when the snapshot is deleted.
This might make administration more difficult since the file
is not visible in the file system. If this option is not
specified, the backing-store files should be removed manually
after the snapshot is deleted.


chunksize=n [k,m,g]

Uses n for the chunk size. Chunk size is the granularity of
the data that is sent to the backing store.

Specify chunksize in the following units: k for kilobytes, m
for megabytes, or g for gigabytes. By default, chunk size is
four times the block size of the file system (typically 32k).


maxsize=n[k,m,g]

Does not allow the sum of the sizes of the backing-store
file(s) to exceed n, where n is the unit specified. The
snapshot is deleted automatically when the sum of the sizes
of the backing-store file(s) exceeds maxsize.

Specify maxsize in the following units: k for kilobytes, m
for megabytes, or g for gigabytes.


raw

Displays to standard output the name of the raw device
instead of the block device when a snapshot is created. The
block device is printed by default (when raw is not
specified). This option makes it easier to embed fssnap
commands in the command line for commands that require the
raw device instead. Both devices are always created. This
option affects only the output.

OPERANDS

The following operands are supported:

mount-point

The directory where the file system resides.


special

The physical device for the file system, such as
/dev/dsk/c0t0d0s7.

EXAMPLES

Example 1: Creating a Snapshot of a File System

The following example creates a snapshot of a file system. The block
special device created for the snapshot is /dev/fssnap/0.


# fssnap -F ufs -o backing-store=/var/tmp /export/home
/dev/fssnap/0

Example 2: Backing Up a File System Snapshot Without Having To Unmount

the File System


The following example backs up a file system snapshot without having
to unmount the file system. Since ufsdump requires the path to a raw
device, the raw option is used. The /export/home file system snapshot
is removed in the second command.


# ufsdump 0uf /dev/rmt/0 `fssnap -F ufs
-o raw,bs=/export/snap /export/home`
<output from ufsdump>
# fssnap -F ufs -d /export/home

Example 3: Backing Up a File System

When backing up a file system, do not let the backing-store file(s)
exceed 400 Mbytes. The second command removes the /export/home file
system snapshot.


# ufsdump 0uf /dev/rmt/0 `fssnap -F ufs
-o maxsize=400m,backing-store=/export/snap,raw
/export/home`
# fssnap -F ufs -d /export/home

Example 4: Performing an Incremental Dump of a Snapshot

The following example uses ufsdump to back up a snapshot of /var.
Note the use of the N option to ufsdump, which writes the name of the
device being dumped, rather than the name of the snapshot device, to
/etc/dumpdates file. See ufsdump(8) for details on the N flag.


# ufsdump lfNu /dev/rmt/0 /dev/rdsk/c0t3d0s2 `fssnap -F ufs
-o raw,bs=/export/scratch,unlink /var`

Example 5: Finding Out What Snapshots Currently Exist

The following command displays the currently existing snapshots.


# fssnap -i
0 /src
1 /export/home
<output continues>

Example 6: Mounting a File System Snapshot

The following example creates a file system snapshot. After you
create a file system snapshot, mount it on /tmp/mount for temporary
read-only access.


# fssnap -F ufs -o backing-store=/nfs/server/scratch /export/home
/dev/fssnap/1
# mkdir /tmp/mount
# mount -F ufs -o ro /dev/fssnap/1 /tmp/mount

Example 7: Creating a File System Snapshot and Unlinking the Backing-

store File


The following example creates a file system snapshot and unlinks the
backing-store file. After creating a file system snapshot and
unlinking the backing-store file, check the state of the snapshot.


# fssnap -o bs=/scratch,unlink /src
/dev/fssnap/0
# fssnap -i /src
Snapshot number : 0
Block Device : /dev/fssnap/0
Raw Device : /dev/rfssnap/0
Mount point : /src
Device state : active
Backing store path : /scratch/snapshot2 <UNLINKED>
Backing store size : 192 KB
Maximum backing store size : Unlimited
Snapshot create time : Sat May 06 10:55:11 2000
Copy-on-write granularity : 32 KB

Example 8: Displaying the Size and Location of the Backing-store

File(s) and the Creation Time for the Snapshot


The following example displays the size of the backing-store file(s)
in bytes, the location of the backing store, and the creation time
for the snapshot of the /test file system.


# fssnap -i -o backing-store-len,backing-store,createtime /test
196608
/snapshot2
Sat May 6 10:55:11 2000


Note that if there are multiple backing-store files stored in
/snapshot2, they will have names of the form file (for the first
file), file.1, file.2, and so forth.

EXIT STATUS

The following exit values are returned:

0

Successful completion.


>0

An error occurred.


The script-readable output mode is a stable interface that can be
added to, but will not change. All other interfaces are subject to
change.

SEE ALSO

mlock(3C), attributes(7)

NOTES

The fssnap device files should be treated like a regular disk block
or character device.


The association between a file system and the snapshot is lost when
the snapshot is deleted or the system reboots. Snapshot persistence
across reboots is not supported.


To avoid unnecessary performance impacts, perform the snapshot and
system backup when the system is least active.


It is not possible to perform a snapshot of a file system if any of
the following conditions are true:

o The file system is in use by system accounting

o The file system contains a local swap file

o The file system is used as backing store by an application
that uses mlock(3C) to lock its pages. Typically, these
are real time applications, such as ntpd.


These conditions result in fssnap being unable to write lock the file
system prior to performing the snapshot.

May 13, 2017 FSSNAP_UFS(8)