GPFS: Administration and Programming Reference - IRA Home

mmcrsnapshot Command
Name
mmcrsnapshot – Creates a snapshot of an entire GPFS file system at a single point in time.
Synopsis
mmcrsnapshot Device Directory
Description
Use the mmcrsnapshot command to create a snapshot of an entire GPFS file system at a single point in
time.
A snapshot is a copy of the changed user data in the file system. System data and existing snapshots are
not copied. The snapshot function allows a backup or mirror program to run concurrently with user updates
and still obtain a consistent copy of the file system as of the time the copy was created. Snapshots are
exact copies of changed data in the active files and directories of a file system. Snapshots of a file system
are read-only and they appear in a .snapshots directory located in the file system root directory. The files
and attributes of the file system may be changed only in the active copy.
There is a maximum limit of 31 snapshots per file system. Snapshots may be deleted only by issuing the
mmdelsnapshot command. The .snapshots directory cannot be deleted, though it can be renamed with
the mmsnapdir command using the -s option.
If the mmcrsnapshot command is issued while a conflicting command is running, the mmcrsnapshot
command waits for that command to complete. If the mmcrsnapshot command is running while a
conflicting command is issued, the conflicting command waits for the mmcrsnapshot command to
complete. Conflicting operations include:
1. Other snapshot commands
2. Adding, deleting, replacing disks in the file system
3. Rebalancing, repairing, reducing disk fragmentation in a file system
If quorum is lost before the mmcrsnapshot command completes, the snapshot is considered partial and
will be deleted when quorum is achieved again.
Because snapshots are not full, independent copies of the entire file system, they should not be used as
protection against media failures. For protection against media failures, see General Parallel File System:
Concepts, Planning, and Installation Guide and search on recoverability considerations.
For more information on snapshots, see Creating and maintaining snapshots of GPFS file systems in
General Parallel File System: Advanced Administration Guide.
Parameters
Device
The device name of the file system for which the snapshot is to be created. File system names
need not be fully-qualified. fs0 is just as acceptable as /dev/fs0.
This must be the first parameter.
Directory
Specifies the name given to the snapshot. The name appears as a subdirectory of the .snapshots
directory in the file system root. Each snapshot must have a unique name.
Chapter 8. GPFS commands 131