Understanding mirrored aggregates

About the SyncMirror feature

The SyncMirror software creates aggregates or traditional volumes that consist of two copies of the same WAFL file system. The two copies, known as plexes, are simultaneously updated; therefore, the copies are always identical.

Advantages of SyncMirror

A SyncMirror relationship between two aggregates or traditional volumes provides a high level of data availability because the two plexes are physically separated on different shelves and the shelves are connected to the storage system with separate cables and adapters. Each plex has its own collection of spare disks.

Physical separation of the plexes protects against data loss in the case of a double-disk error or loss of disk connectivity. The unaffected plex continues to serve data while you fix the cause of the failure. Once fixed, the two plexes can be resynchronized and the mirror relationship reestablished.

Another advantage of mirrored plexes is faster rebuild time.

In contrast, if a SnapMirrored aggregate or traditional volume goes down, its SnapMirror partner cannot automatically take over the file serving functions and can only restore data to its condition at the time the last Snapshot copy was created (you must issue commands to make the partner's data available).

Disadvantage of SyncMirror

The disadvantage of SyncMirror is that a mirrored aggregate or traditional volume requires twice as many disks as an unmirrored aggregate or traditional volume. Each of the two plexes requires a full set of disks. For example, you need 200 GB of disk space to mirror a 100-GB traditional volume--100 GB for each plex of the mirrored traditional volume, or 2,880 GB of disk space to mirror a 1,440-GB aggregate.

Prerequisites for using mirrored volumes

The following are prerequisites for using mirrored aggregates or traditional volumes:

You must purchase and enable a SyncMirror license. You must enter this key to receive important product information prior to enabling the synchronous SnapMirror feature.
To enable the synchronous SnapMirror feature, use the license add command, and enter one of the following codes:

Platform	License
Data ONTAP storage system	KZZTWOJ
NearStore®	TXKMEAK
Gateway	PLFQNUJ

You must have an F800 series storage system, F800 series cluster, or better.
Note
FAS200 series storage systems and NearStore storage systems do not support SyncMirror configurations.
You must have DS14 or FC9 disk shelves.
You must connect disk shelves to the storage system in a configuration that supports mirrored volumes. For more information about connecting disk shelves to support mirrored aggregates or traditional volumes, see the Storage Management Guide, System Configuration Guide, Cluster Guide, Fibre Channel DiskShelf14 Hardware Guide, and Fibre Channel StorageShelf FC9 Hardware Guide.
Note
The SyncMirror function is not supported on cluster configurations that use the Servernet cluster card.

What mirrored aggregates and traditional volumes are

A mirrored aggregate or traditional volume is a single WAFL storage file system with two physically separated and synchronously up-to-date copies on disks. These copies are called plexes. Data ONTAP automatically names the first plex plex0 and the second plex plex1.

Each plex is a physical copy of the same WAFL file system and consists of one or more RAID groups. Because SyncMirror duplicates complete WAFL file systems, you cannot use the SyncMirror feature with a FlexVol volume--only aggregates (including all contained FlexVol volumes) or traditional volumes are supported. The following diagram illustrates the concept of the plexes in a traditional SyncMirror relationship.

The following diagram illustrates the concept of the plexes in an aggregate SyncMirror relationship. Note that each plex includes a copy of the FlexVol volumes contained in the aggregate.

Example

The mirrored volume vol0 has two plexes, vol0/plex0 and vol0/plex1.

Where mirrored volume plexes get their disks

When a mirrored volume is created, Data ONTAP separates spare disks from a collection of disks, called a disk pool, into two disk pools, pool0 and pool1. When assigning a disk to a pool, Data ONTAP determines the shelf for the disk and makes sure that the disks in pool0 are from different shelves than the disks in pool1. So before enabling SyncMirror, make sure disks are installed in at least two shelves and the shelves are connected to the storage system with separate cables and adapters. Disk pools must be physically separate to ensure high availability of the mirrored volume.

Disks from pool0 are used to create plex0 while disks from pool1 are used to create plex1.

Plex pool assignment

Plexes local to the host node in a cluster must be connected to the disk pool named pool0. Pool0 consists of the storage attached to host adapters in slots 3 through 7. Pool rules for MetroCluster configurations that use switches are different. See the System Configuration Guide for more information about storage system slot assignments.