Peer snapshot -psnap

The peer snapshot function provides an application-consistent point-in-time copy of data in the cache fileset and at home enabling cross-site data consistency points.

When you take a peer snapshot it creates a snapshot in the cache fileset then queues a snapshot to be executed at the home. When all of the queued requests outstanding at the time the snap was performed in cache have been flushed to the home fileset so that the home data is consistent with cache, a snapshot of the corresponding home fileset is performed. The result is a pair of peer snapshots, one at the cache and one at the home that refer to the same consistent copy.

Peer snapshots are created using the mmpsnap command from the cache. To create a peer snapshot the cache fileset must be in the active state. The last successful snapshot is saved and can be viewed by running the mmlssnapshot command on either side. Multiple outstanding peer snapshots can be queued on the gateway. Peer snapshots must not be deleted by running the mmdelsnapshot command, use the mmpsnap command to ensure that both the cache and home snapshots are removed. The mmpsnap command works only if the home cluster has run the mmafmconfig enable ExportPath command. The mmpsnap command can be used only in an SW cache. It cannot be used for RO, IW, or LU caches.

If the cache fileset is disconnected from the home fileset when the cache snapshot is created, the cache records that the peer snapshot on the home fileset has not been created. When connection is restored, it attempts to create the snapshot.

Peer snapshots are not allowed on a SW cache that uses the NSD protocol for communicating with home.

To create a fileset level snapshot in cache of a single-writer fileset called sw in file system fs1, run the following command:

mmpsnap fs1 create -j sw

The system displays the following output:

Writing dirty data to disk.
Quiescing all file system operations.
Writing dirty data to disk again.
Snapshot psnap-13882361812785649740-C0A80E85:4F44B305-59-12-03-01-02-27-28 created with id 8.
Snapshot psnap-13882361812785649740-C0A80E85:4F44B305-59-12-03-01-02-27-28 created at the satellite. 
Core snapshot has been queued.

To view the snapshot run the following command:

mmlssnapshot fs1 -j sw

The system displays the following output:

Snapshots in file system fs1:
Directory SnapId Status Created Fileset
psnap-13882361812785649740-C0A80E85:4F44B305-59-12-03-01-02-27-28 8 Valid Thu Mar 1 02:27:29 2012 sw

To show that the snapshot is also created at home, run the following command:

mmlssnapshot fs1

The system displays the following output:

Snapshots in file system fs1:
Directory SnapId Status Created Fileset
psnap-13882361812785649740-C0A80E85:4F44B305-59-12-03-01-02-27-28 8 Valid Thu Mar 1 02:23:16 2012