SnapMirror Synchronous (SM-S) technology is supported on all ETERNUS HX and ETERNUS AX 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\ONTAP 9.12.1
ETERNUS AX	80	160	200	400
ETERNUS HX	40	80	80	80

Supported features

ONTAP 9.12.1 supports non-disruptive SnapMirror Synchronous operations (NDO) on ETERNUS AX/ASA platforms, only. Support for non-disruptive operations enables you to perform many common maintenance tasks without scheduling down time. Operations supported include takeover and giveback, and volume move, provided that a single node is surviving among each of the two clusters.

The following features are supported for SnapMirror Synchronous technology in ONTAP 9.10.1; provided all nodes in the source and destination cluster are running ONTAP 9.10.1:

NFSv4.2
NVMe/TCP

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

SnapMirror Synchronous supports source and destination volumes on FabricPool aggregates with a tiering policy of None, Snapshot, or Auto. The destination volume in a FabricPool aggregate cannot be set to All tiering policy.

The following features are supported for SnapMirror Synchronous technology in ONTAP 9.7:

Replication of application-created Snapshot copies
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
LUN clones and NVMe namespace clones
LUN clones backed by application-created Snapshot copies are also supported.
SVM DR
- A SnapMirror Synchronous source can also be a SVM DR source, for example, a fan-out configuration with SM-S as one leg and SVM DR as the other.
- A SnapMirror Synchronous source cannot be an SVM DR destination because SM-S does not support cascading a DP source.
  You must release the synchronous relationship before performing an SVM DR flip resync in the destination cluster.
- A SnapMirror Synchronous destination cannot be an SVM DR source because SVM DR does not support replication of DP volumes.
  A flip resync of the synchronous source would result in the SVM DR excluding the DP volume in the destination cluster.
NFSv4.0 and NFSv4.1
SMB 2.0 or later
Mixed protocol access (NFSv3 and SMB)
Antivirus on the primary volume of the SnapMirror Synchronous relationship
Hard or soft quotas on the primary volume of the SnapMirror Synchronous relationship
The quota rules are not replicated to the destination; therefore, the quota database is not replicated to the destination.
FPolicy on the primary volume of the SnapMirror Synchronous relationship
SnapMirror Synchronous mirror-mirror cascade
The relationship from the destination volume of the SnapMirror Synchronous relationship must be an asynchronous SnapMirror relationship.
Timestamp parity between source and destination volumes for NAS
Removal of high metadata operation frequency limitation
Security for sensitive data in-transit using TLS 1.2 encryption
Clone autodelete

Beginning in ONTAP 9.13.1, NDMP is supported with SnapMirror Synchronous. 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.

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
LUN clone/file clone
MetroCluster configurations
Mixed SAN and NVMe access
LUNs and NVMe namespaces are not supported on the same volume or SVM.
SnapCenter
SnapLock volumes
SnapRestore
Tamperproof Snapshot copies
Tape backup or restore using dump and SMTape on the destination volume
Tape based restore to the source volume
Throughput floor (QoS Min) for source volumes
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.