FSSNAP_UFS(8) Maintenance Procedures FSSNAP_UFS(8)


fssnap_ufs - create a temporary snapshot of a UFS file system


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


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.


The following options are supported:


Deletes the snapshot associated with the given file system.


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

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


Display the snapshot number.


Display the block device path.


Display the raw device path.


Display the mount point of the master file system.


Display the state of the snapshot device.


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.


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


Display the maxsize value specified for the backing-store


Display the time that the snapshot was created.


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.


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.


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

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).


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.


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.


The following operands are supported:


The directory where the file system resides.


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


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

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
# 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

# fssnap -F ufs -o backing-store=/nfs/server/scratch /export/home
# 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


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
# 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
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.


The following exit values are returned:


Successful completion.


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.


mlock(3C), attributes(7)


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)