SnapMirror Synchronous disaster recovery basics

Beginning with ONTAP 9.5, SnapMirror Synchronous (SM-S) technology is supported on all FAS and AFF platforms that have at least 16 GB of memory and on all ONTAP Select platforms. SnapMirror Synchronous technology is a capacity based, per-node, licensed feature that provides synchronous data replication at the volume level.

This functionality addresses the regulatory and national mandates for synchronous replication in financial, healthcare, and other regulated industries where zero data loss is required.

The limit on the number of SnapMirror Synchronous replication operations per node depends on the controller model.

Platform Number of SnapMirror Synchronous operations that are allowed per node
AFF 40
FAS 20
ONTAP Select 10

Supported features

SnapMirror Synchronous technology supports the NFSv3, FC, and iSCSI protocols over all networks for which the latency does not exceed 10ms.

The following features are supported for SnapMirror Synchronous technology in ONTAP 9.6; provided all nodes in the source and destination cluster are running ONTAP 9.6:

Unsupported features

The following features are not supported with Synchronous SnapMirror relationships:

Modes of operation

SnapMirror Synchronous has two modes of operation based on the type of the SnapMirror policy used:

Sync mode

In Sync mode, an I/O to primary storage is first replicated to secondary storage. Then the I/O is written to primary storage, and acknowledgment is sent to the application that issued the I/O. If the write to the secondary storage is not completed for any reason, the application is allowed to continue writing to the primary storage. When the error condition is corrected, SnapMirror Synchronous technology automatically resynchronizes with the secondary storage and resumes replicating from primary storage to secondary storage in Synchronous mode.

In Sync mode, RPO=0 and RTO is very low until a secondary replication failure occurs at which time RPO and RTO become indeterminate, but equal the time to repair the issue that caused secondary replication to fail and for the resync to complete.

StrictSync mode

SnapMirror Synchronous can optionally operate in StrictSync mode. If the write to the secondary storage is not completed for any reason, the application I/O fails, thereby ensuring that the primary and secondary storage are identical. Application I/O to the primary resumes only after the SnapMirror relationship returns to the InSync status. If the primary storage fails, application I/O can be resumed on the secondary storage, after failover, with no loss of data.

In StrictSync mode RPO is always zero, and RTO is very low.

Relationship status

The status of a SnapMirror Synchronous relationship is always in the InSync status during normal operation. If the SnapMirror transfer fails for any reason, the destination is not in sync with the source and can go to the OutofSync status.

For SnapMirror Synchronous relationships, the system automatically checks the relationship status (InSync or OutofSync) at a fixed interval. If the relationship status is OutofSync, ONTAP automatically triggers the auto resync process to bring back the relationship to the InSync status. Auto resync is triggered only if the transfer fails due to any operation, such as unplanned storage failover at source or destination or a network outage. User-initiated operations such as snapmirror quiesce and snapmirror break do not trigger auto resync.

If the relationship status becomes OutofSync for a SnapMirror Synchronous relationship in the StrictSync mode, all I/O operations to the primary volume are stopped. The OutofSync state for SnapMirror Synchronous relationship in the Sync mode is not disruptive to the primary and I/O operations are allowed on the primary volume.