SnapMirror unified replication basics

How unified data protection relationships are initialized​

As with SnapMirror, unified data protection performs a baseline transfer the first time you invoke it. The SnapMirror policy for the relationship defines the contents of the baseline and any updates.

A baseline transfer under the default unified data protection policy MirrorAndVault makes a Snapshot copy of the source volume, then transfers that copy and the data blocks it references to the destination volume. Like SnapVault, unified data protection does not include older Snapshot copies in the baseline.

How unified data protection relationships are updated​

At each update under the MirrorAndVault policy, SnapMirror creates a Snapshot copy of the source volume and transfers that Snapshot copy and any Snapshot copies that have been made since the last update, provided they have labels matching the labels defined in the Snapshot policy rules. In the following output from the snapmirror policy show command for the MirrorAndVault policy, note the following:

• Create Snapshot is true, indicating that MirrorAndVault creates a Snapshot copy when SnapMirror updates the relationship.
• MirrorAndVault has rules sm_created, daily, and weekly, indicating that both the Snapshot copy created by SnapMirror and the Snapshot copies with matching labels on the source are transferred when SnapMirror updates the relationship.

cluster_dst::> snapmirror policy show -policy MirrorAndVault -instance

                     Vserver: vs0
      SnapMirror Policy Name: MirrorAndVault
      SnapMirror Policy Type: mirror-vault
                Policy Owner: cluster-admin
                 Tries Limit: 8
           Transfer Priority: normal
   Ignore accesstime Enabled: false
     Transfer Restartability: always
 Network Compression Enabled: false
             Create Snapshot: true
                     Comment: A unified Synchronous SnapMirror and SnapVault policy for
                              mirroring the latest file system and daily and weekly snapshots.
       Total Number of Rules: 3
                  Total Keep: 59
                       Rules: SnapMirror Label     Keep  Preserve Warn Schedule Prefix
                              ----------------     ----  -------- ---- -------- ------
                              sm_created              1  false       0 -        -
                              daily                   7  false       0 -        -
                              weekly                 52  false       0 -        -

Unified7year policy​

The preconfigured Unified7year policy works exactly the same way as MirrorAndVault, except that a fourth rule transfers monthly Snapshot copies and retains them for seven years.

                       Rules: SnapMirror Label     Keep  Preserve Warn Schedule Prefix
                              ----------------     ----  -------- ---- -------- ------
                              sm_created              1  false       0 -        -
                              daily                   7  false       0 -        -
                              weekly                 52  false       0 -        -
                              monthly                84  false       0 -        -

How to protect against possible data corruption​

Unified replication limits the contents of the baseline transfer to the Snapshot copy created by SnapMirror at initialization. At each update, SnapMirror creates another Snapshot copy of the source and transfers that Snapshot copy and any new Snapshot copies that have labels matching the labels defined in the Snapshot policy rules.

You can protect against the possibility that an updated Snapshot copy is corrupted by creating a copy of the last transferred Snapshot copy on the destination. This local copy is retained regardless of the retention rules on the source, so that even if the Snapshot originally transferred by SnapMirror is no longer available on the source, a copy of it will be available on the destination.

When to use unified data replication​

You need to weigh the benefit of maintaining a full mirror against the advantages that unified replication offers in reducing the amount of secondary storage, limiting the number of baseline transfers, and decreasing network traffic.

The key factor in determining the appropriateness of unified replication is the rate of change of the active file system. A traditional mirror might be better suited to a volume holding hourly Snapshot copies of database transaction logs, for example.