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.
Sync-mirrored volumes provide a high level of data availability by physically separating the two plexes of a sync-mirrored volume. Additionally, each plex has its own collection of spare disks that are separated from the other plex`s spare disks.
This physical separation of the plexes protects against loss of data if a double-disk error or loss of disk connectivity occurs. 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 mirrored 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 of the last snapshot creation.
The disadvantage of SyncMirror is that you must use twice as many disks on a mirrored volume as you would on 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.
The following are prerequisites for using mirrored volumes:
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.
A mirrored volume is a single volume with two physically separated and synchronously up-to-date copies on disks.
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.
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.
The mirrored volume vol0 has two plexes, vol0/plex0 and vol0/plex1.
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.
When you configure your filer to use mirrored volumes, you connect the shelves to the filer in a way that physically separates spare disks into two pools. You do this by using separate cables and adapters to connect separate shelves to the filer.
After you configure your filer and disk shelves and enable the mirrored volume feature, Data ONTAP divides spare disks into two different 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 from one another to ensure high availability of the mirrored volume.
The advantage of mirrored volumes over unmirrored volumes is that there 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.
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Part No. 210-00325
File: syncmir2.htm
Last updated: 01/14/03 17:51:13