fssnap_ufs(1M)
NAME
fssnap_ufs - create a temporary snapshot of a UFS file sys-
tem
SYNOPSIS
fssnap [-F] [ufs] [generic-options] -o backing-
store=path,[specific-options] mount-point | special
fssnap [-F ufs] [-d] [generic-options] [-o specific-
options] mount-point | special
fssnap [-F ufs] [-i] [generic-options] [-o specific-
options] mount-point | special
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 is one in which the snapshot subsys-
tem saves old file system data before it is overwritten. The
destination path must have enough free space to hold the
backing-store file, whose size varies with the amount of
activity on the file system. This location must be different
from the file system that is being captured in a snapshot.
The backing-store file can reside on any type of file sys-
tem, 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 interna-
tionalized 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 sys-
tem.
state Display the state of the snapshot device.
backing-store
Display the location of the backing-store file.
backing-store-len
Display the size of the backing-store file.
maxsize
Display the max size of the backing-store file.
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 (bs), which is
required.
backing-store=path
Uses path as the backing-store file. path must
not reside on the file system that is being cap-
tured in a snapshot.
path must exist, and must be either a directory
or a regular file. If path is a directory, then
a temporary file is created and held open. That
device is then used as-is. The 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 sys-
tem. 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 size of the backing-store
file to exceed n, where n is the unit specified.
The snapshot is deleted automatically when the
backing-store file 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 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
/dev/rdsk/c0t3d0s2. 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(1M) for details on the N flag.
# ufsdump lfNu /dev/rmt/0 /dev/rdsk/c0t3d0s2 `fssnap -F ufs
-o raw,bs=/export/scratch,unlink /dev/rdsk/c0t3d0s2`
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 and the Creation Time for the Snapshot
The following example displays the size of the backing-store
file 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
EXIT STATUS
The following exit values are returned:
0 Successful completion.
>0 An error occurred.
ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| Availability | SUNWcsu |
|_____________________________|_____________________________|
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.
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 currently sup-
ported.
To avoid unnecessary performance impacts, perform the
snapshot and system backup when the system is least active.
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