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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:
-d
-i
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
blockdevname
rawdevname
mountpoint
state
backing-store
backing-store-len
maxsize
createtime
chunksize
-o specific-options
backing-store=path
This option can be abbreviated as bf=path or bs=path.
unlink
chunksize=n [k,m,g]
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]
Specify maxsize in the following units: k for kilobytes, m for megabytes, or g for gigabytes.
raw
The following operands are supported:
mount-point
special
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(1M) 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.
The following exit values are returned:
0
>0
See attributes(5) for descriptions of the following attributes:
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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.
xntpd(1M), mlock(3C), attributes(5)
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 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:
These conditions result in fssnap being unable to write lock the file system prior to performing the snapshot.
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