Understanding mirrored volumes

About the SyncMirror feature

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

Advantages of SyncMirror

Volumes in a SyncMirror relationship provide a high level of data availability because the two plexes are physically separated. 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 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 was created (you must issue commands to make the partner's data available).

Disadvantage of SyncMirror

The disadvantage of SyncMirror is that a mirrored volume requires twice as many disks as an unmirrored 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 volume--100 GB for each plex of the mirrored volume.

Prerequisites for using mirrored volumes

The following are prerequisites for using mirrored volumes:

You must purchase and enable a SyncMirror license.
You must have an F800 series filer, F800 series cluster, or better.
You must have DS14 or FC9 disk shelves.
You must connect disk shelves to the filer in a configuration that supports mirrored volumes. For more information about connecting disk shelves to support mirrored volumes, see the System Configuration Guide, F800 Series 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. The SyncMirror function is supported on clusters using a Troika cluster card.

What mirrored volumes are

A mirrored volume is a single volume with two physically separated and synchronously up-to-date copies on disks.

What a SyncMirror file system is called

The two copies of a mirrored volume are called plexes. Each plex is a physical copy of the same file system and consists of one or more RAID groups. The following diagram illustrates the concept of a plex in a mirrored volume.

Plex naming convention

The convention used by Data ONTAP to name plexes is as follows:

vol-name/plexplexid

vol-name is the name of the volume.

plexid is the number of the plex and is assigned by Data ONTAP.

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 takes spare disks from a collection of disks, called a disk pool, to create one plex and spare disks from another disk pool to create the other plex.

Note
To enable a filer for mirrored volumes, you must configure the filer and disk shelves to use two disk pools.

How disk pools are created

When you configure your filer to use mirrored volumes, you physically separate spare disks in two pools. Use separate cables and adapters to connect separate shelves to the filer.

After you configure your filer and disk shelves and enable SyncMirror, Data ONTAP divides spare disks into two logical collections called disk pools. When Data ONTAP creates disk pools, it uses your configuration to ensure that one disk pool is physically separate from the other disk pool. Disk pools must be physically separate to ensure high availability of the mirrored volume.

Advantage of mirrored volumes

The advantage of mirrored volumes is is a higher availability of data. Mirrored data exists on separate shelves connected to the filer by separate cables and adapters to avoid single points of failure. Because of this separation and because the two plexes of a mirrored volume contain the same data at the same time, one plex can fail and you will still have access to the data through the other plex.