SnapMirror Synchronous (SM-S) technology is supported on all ETERNUS HX series and ETERNUS AX/AC series that have at least 16 GB of memory. SnapMirror Synchronous technology is a 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 HA pair depends on the controller model.

The following table lists the number of SnapMirror Synchronous operations that are allowed per HA pair according to platform type and ONTAP release.

Platform	Releases earlier than ONTAP 9.9.1	ONTAP 9.9.1	ONTAP 9.10.1	ONTAP 9.11.1 through ONTAP 9.14.1
ETERNUS AX	80	160	200	400
ASA	80	160	200	400
ETERNUS HX	40	80	80	80

Supported features

The following table indicates the features supported with SnapMirror Synchronous and the ONTAP releases in which support is available.

Feature	Release first supported	Additional information
Application-created Snapshot copy replication	ONTAP 9.7	If a Snapshot copy is tagged with the appropriate label at the time of the `snapshot create` operation, using the CLI or the ONTAP API, SnapMirror Synchronous replicates the Snapshot copies, both user created or those created with external scripts, after quiescing the applications. Scheduled Snapshot copies created using a Snapshot policy are not replicated.
FC-NVMe	ONTAP 9.7
File clones	ONTAP 9.7
LUN clones and NVMe namespace clones	ONTAP 9.7
LUN clones backed by application-created Snapshot copies	ONTAP 9.7
NDMP/NDMP restore	ONTAP 9.13.1	Both the source and destination cluster must be running ONTAP 9.13.1 or later to use NDMP with SnapMirror Synchronous. For more information, see Transfer data using ndmp copy.
Non-disruptive SnapMirror Synchronous operations (NDO) on ETERNUS AX/AC series/ASA platforms, only.	ONTAP 9.12.1	Support for non-disruptive operations enables you to perform many common maintenance tasks without scheduling down time.
NFS v4.2	ONTAP 9.10.1
NVMe/TCP	9.10.1
Single file and partial file restore	ONTAP 9.13.1	A SnapMirror Synchronous source cannot be an SVM disaster recovery destination because SnapMirror Synchronous does not support cascading a data protection source. You must release the synchronous relationship before performing an SVM disaster recovery flip resync in the destination cluster. A SnapMirror Synchronous destination cannot be an SVM disaster recovery source because SVM disaster recovery does not support replication of DP volumes. A flip resync of the synchronous source would result in the SVM disaster recovery excluding the DP volume in the destination cluster.
Tape-based restore to the source volume	ONTAP 9.13.1

Feature

Release first supported

Additional information

Application-created Snapshot copy replication

ONTAP 9.7

If a Snapshot copy is tagged with the appropriate label at the time of the snapshot create operation, using the CLI or the ONTAP API, SnapMirror Synchronous replicates the Snapshot copies, both user created or those created with external scripts, after quiescing the applications. Scheduled Snapshot copies created using a Snapshot policy are not replicated.

FC-NVMe

ONTAP 9.7

File clones

ONTAP 9.7

LUN clones and NVMe namespace clones

ONTAP 9.7

LUN clones backed by application-created Snapshot copies

ONTAP 9.7

NDMP/NDMP restore

ONTAP 9.13.1

Both the source and destination cluster must be running ONTAP 9.13.1 or later to use NDMP with SnapMirror Synchronous. For more information, see Transfer data using ndmp copy.

Non-disruptive SnapMirror Synchronous operations (NDO) on ETERNUS AX/AC series/ASA platforms, only.

ONTAP 9.12.1

Support for non-disruptive operations enables you to perform many common maintenance tasks without scheduling down time.

NFS v4.2

ONTAP 9.10.1

NVMe/TCP

9.10.1

Single file and partial file restore

ONTAP 9.13.1

A SnapMirror Synchronous source cannot be an SVM disaster recovery destination because SnapMirror Synchronous does not support cascading a data protection source.
You must release the synchronous relationship before performing an SVM disaster recovery flip resync in the destination cluster.
A SnapMirror Synchronous destination cannot be an SVM disaster recovery source because SVM disaster recovery does not support replication of DP volumes.
A flip resync of the synchronous source would result in the SVM disaster recovery excluding the DP volume in the destination cluster.

Tape-based restore to the source volume

ONTAP 9.13.1

Unsupported features

The following features are not supported with Synchronous SnapMirror relationships:

Consistency groups
DP_Optimized (DPO) systems
FlexGroup volumes
FlexCache volumes
Global throttling
In a fan-out configuration, only one relationship can be a SnapMirror Synchronous relationship; all the other relationships from the source volume must be asynchronous SnapMirror relationships.
LUN move
MetroCluster configurations
Mixed SAN and NVMe access
LUNs and NVMe namespaces are not supported on the same volume or SVM.
SnapCenter
SnapLock volumes
Tamperproof Snapshot copies
Tape backup or restore using dump and SMTape on the destination volume
Throughput floor (QoS Min) for source volumes
Volume SnapRestore
VVol

Modes of operation

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

Sync mode
In Sync mode, application I/O operations are sent in parallel to the primary and secondary
storage systems. 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.