Understanding aggregates

Aggregate management

To support the differing security, backup, performance, and data sharing needs of your users, you can group the physical data storage resources on your storage system into one or more aggregates.

Each aggregate possesses its own RAID configuration, plex structure, and set of assigned disks. Within each aggregate you can create one or more FlexVol volumes--the logical file systems that share the physical storage resources, RAID configuration, and plex structure of that common containing aggregate.

For example, you can create a large aggregate with large numbers of disks in large RAID groups to support multiple FlexVol volumes, maximize your data resources, provide the best performance, and accommodate SnapVault backup.

You can also create a smaller aggregate to support FlexVol volumes that require special functions like SnapLock non-erasable data storage.

An unmirrored aggregate

In the following diagram, the unmirrored aggregate, arbitrarily named aggrA by the user, consists of one plex, which is made up of three double-parity RAID groups, automatically named rg0, rg1, and rg2 by Data ONTAP.

Notice that RAID-DP requires that both a parity disk and a double parity disk be in each RAID group. In addition to the disks that have been assigned to RAID groups, there are eight hot spare disks in the pool. In this diagram, two of the disks are needed to replace two failed disks, so only six will remain in the pool.

A mirrored aggregate

Consists of two plexes, which provides an even higher level of data redundancy via RAID-level mirroring. For an aggregate to be enabled for mirroring, the storage system must have a SyncMirror license for syncmirror_local or cluster_remote installed and enabled, and the storage system's disk configuration must support RAID-level mirroring.

When SyncMirror is enabled, all the disks are divided into two disk pools, and a copy of the plex is created. The plexes are physically separated (each plex has its own RAID groups and its own disk pool), and the plexes are updated simultaneously. This provides added protection against data loss if there is a double-disk failure or a loss of disk connectivity, because the unaffected plex continues to serve data while you fix the cause of the failure. Once the plex that had a problem is fixed, you can resynchronize the two plexes and reestablish the mirror relationship. For more information about snapshots, SnapMirror, and SyncMirror, see the Data Protection Online Backup and Recovery Guide.

In the following diagram, SyncMirror is enabled and implemented, so plex0 has been copied and automatically named plex1 by Data ONTAP. Plex0 and plex1 contain copies of one or more file systems. In this diagram, thirty-two disks had been available prior to the SyncMirror relationship being initiated. After initiating SyncMirror, each pool has its own collection of sixteen hot spare disks.

When you create an aggregate, Data ONTAP assigns data disks and parity disks to RAID groups, depending on the options you choose, such as the size of the RAID group (based on the number of disks to be assigned to it) or the level of RAID protection.

Choosing the right size and the protection level for a RAID group depends on the kind of data that you intend to store on the disks in that RAID group. For more information about planning the size of RAID groups, see Size of RAID groups and Chapter 4, RAID Protection of Data.