SnapMirror overview

What SnapMirror is

The Data ONTAP SnapMirror feature enables an administrator to mirror snapshot images from a source volume or qtree to a partner destination volume or qtree, thus replicating source object data on destination objects at regular intervals. You can access the information on the destination volume or qtree to

Provide users quick access to mirrored data in the event of a disaster that makes the source volume or qtree unavailable.
Update the source to recover from disaster, data corruption (qtrees only), or user error
Archive the data to tape
Balance resource loads
Back up or distribute the data to remote sites
Note
This chapter describes how to set up and use SnapMirror on filers configured for network attached storage (NAS). For a description of SnapMirror implementation and management on filers configured for a storage area network (SAN) environment, see the SAN Setup Overview FCP 1.1 Guide.

The components of SnapMirror

The basic SnapMirror deployment consists of the following components.

Source volumes or qtrees

SnapMirror source volumes and qtrees are writable data objects whose data is to be replicated. The source volumes and qtrees are the objects normally visible, accessible, and writable by the filer clients.

Destination volumes or qtrees

The SnapMirror destination volumes and qtrees are read-only objects, usually on a separate filer, to which the source volumes and qtrees are replicated. The destination volumes and qtrees are normally accessed by users only when a disaster takes down the source volumes or qtrees and the administrator uses SnapMirror commands to make the replicated data at the destination accessible and writable.

Source to destination to tape variation

A common variation to the basic SnapMirror backup deployment adds a tape backup of the destination volume. By running a tape backup off the SnapMirror destination volume, you do not subject the heavily user-accessed source volume to the performance degradation, system unavailability, and complexity of a direct tape backup.

Source to tape to destination variation

A SnapMirror deployment that supports SnapMirror replication over low-bandwidth connections accommodates an initial mirroring between a source and destination volume via physically transported tape. Once the large-sized base snapshot replication has been carried out, smaller-sized, incremental snapshot updates can be carried out over the low-bandwidth connection.

Cascading destinations variation

A variation on the basic SnapMirror deployment and function involves a writable source volume replicated to multiple read-only destinations. The function of this deployment is to make a uniform set of data available on a read-only basis to users from various locations throughout a network and to allow for updating that data uniformly at regular intervals.

Note
The cascade deployment is supported for SnapMirror volume replication only. It is not supported for SnapMirror qtree replication.

How SnapMirror works

The SnapMirror feature uses the information in the /etc/snapmirror.conf file and the information you enter via the snapmirror.access option or the /etc/snapmirror.allow file to establish a relationship between a specified source volume or qtree that you want to back up, and the destination volume or qtree where the backup is kept.

The SnapMirror feature does the following:

Creates a snapshot of the data on the source volume or qtree
Copies it to the destination, a read-only volume or qtree
Updates the destination to reflect incremental changes on the source, on the schedule you specify

The result of this process is an online, read-only volume or qtree that contains the same data as the source at the time of the most recent update.

What to use SnapMirror for

You might want to copy or use the data stored on a SnapMirror destination if you are in any of the situations described in When to copy volumes. The additional advantages of SnapMirror make it useful in other data retrieval situations, as described in the following table.

Situation	How to use SnapMirror
Disaster recovery You want to provide immediate access to data after a disaster has made a qtree, volume, or filer unavailable.	You can make the destination writable so clients can use the same data that was on the source volume the last time data was copied.
Data restoration You want to restore lost data on a qtree or volume source from its mirrored qtree or volume SnapMirror partner.	You can temporarily reverse the roles for the source and destination qtrees or volumes and copy the mirrored information back to its source.
Application testing You want to use an application on a database, but you want to test it on a copy of the database in case the application damages the data.	You can make a copy of the database to be used in the application testing to ensure that the data on the source cannot be lost.
Load balancing A large number of users need read-only access to a qtree or volume.	You can copy the data in a qtree or volume to multiple volumes or filers to distribute the load.
Off-loading tape backups You need to reserve all processing and networking resources on a filer for serving NFS and CIFS requests.	After copying data on the source filer, you can back up the data in the destination to tape. This means that the source filer does not have to allocate resources for performing backups.
Remote access to data Users who need read access to a volume are distributed over a large geographical area.	You can copy the source volume to other filers that are geographically closer to the users. Users accessing a local filer can read the data using less resource time than if they connected to a distant filer.

Differences between the vol copy command and SnapMirror

The vol copy command and the SnapMirror feature carry out similar functions. They both enable the administrator to copy snapshots of data from a source filer volume to a destination filer volume. However, they differ in several important ways.

The vol copy feature does not require an additional license.
The SnapMirror feature supports automated and scheduled updates of snapshot data mirrored between the source and destination filer volumes.
The SnapMirror feature supports incremental snapshot updates between source and destination filer volumes.
The SnapMirror feature supports qtree-level replication between the source and destination systems.

Differences between SnapMirror volume replication and qtree replication

You can configure SnapMirror replication for either whole volumes on a filer or individual qtrees on a volume.

SnapMirror volume replication

SnapMirror volume replication has the following characteristics:

SnapMirror volume replication copies a snapshot of a source volume and all its qtrees to a destination.
A destination volume set up for SnapMirror volume replication must first be set to restricted, read-only status.

SnapMirror qtree replication

SnapMirror qtree replication has the following characteristics:

SnapMirror qtree replication copies only the contents of an individual qtree to a destination.
If you need to mirror only the data stored on an individual qtree, then SnapMirror replication of that individual qtree uses less disk space on the filer.
A destination qtree is read-only, but the volume on which it is located must be online and writable.
SnapMirror qtree replication can be set up for a maximum of 255 qtrees on any one volume.

Maximum number of simultaneous replications on a filer

The maximum number of simultaneous SnapMirror replications that each filer model can support is shown in the following table.

Model	Number of Simultaneous Transfers Allowed
F85	4
F87	4
F720	4
F740	4
F760	4
F810	8
F820	8
F840	16
F880	16
F940	16
F960	16
R100	64

How transfers beyond the limit are handled

If more than the allowed number of SnapMirror volume or qtree replications are scheduled to run at the same time, each additional transfer will generate an error message stating that resource limits have been reached. Each transfer beyond the limit will re-attempt to run each following minute until it succeeds, SnapMirror is turned off, or the update is terminated.