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Storage : Aggregates : Understanding aggregates : Protection provided by RAID and SyncMirror
Protection provided by RAID and SyncMirror
Combining RAID and SyncMirror provides protection against more types of disk failures than using RAID alone.
RAID can be used in combination with the Data ONTAP SyncMirror feature, which also offers protection against data loss due to disk or other hardware component failure. SyncMirror protects against data loss by maintaining two copies of the data contained in the aggregate, one in each plex. Any data loss due to disk failure in one plex is repaired by the undamaged data in the other plex.
Note: SyncMirror can be used to provide mirroring of data in array LUNs on third-party storage arrays. However, Data ONTAP provides only RAID0 for data in array LUNs, which does not provide RAID protection. The RAID protection for array LUNs is provided by the third-party storage array.
For more information about SyncMirror, see the Data Protection Online Backup and Recovery Guide.
The following tables outline the differences between using RAID alone and using RAID with SyncMirror.
RAID-DP and SyncMirror
Criteria
RAID-DP alone
RAID-DP with SyncMirror
Failures protected against
Single-disk failure
Double-disk failure within a single RAID group
Multiple-disk failures, as long as no more than two disks within a single RAID group fail
All failures protected against by RAID-DP alone
Any combination of failures protected against by RAID-DP alone in one plex, concurrent with an unlimited number of failures in the other plex
Storage subsystem failures (HBA, cables, shelf), as long as only one plex is affected
Failures not protected against
Three or more concurrent disk failures within a single RAID group
Storage subsystem failures (HBA, cables, shelf) that lead to three or more concurrent disk failures within a single RAID group
Three or more concurrent disk failures in a single RAID group on both plexes
Required disk resources per RAID group
n data disks + 2 parity disks
2 x (n data disks + 2 parity disks)
Performance cost
Almost none
Low mirroring overhead; can improve performance
Additional cost and complexity
None
SyncMirror license and configuration
RAID4 and SyncMirror
Criteria
RAID4 alone
RAID4 with SyncMirror
Failures protected against
Single-disk failure
Multiple-disk failures, as long as no more than one disk within a single RAID group fails
All failures protected against by RAID4 alone
Any combination of failures protected against by RAID4 alone in one plex, concurrent with an unlimited number of failures in the other plex
Storage subsystem failures (HBA, cables, shelf), as long as only one plex is affected
Failures not protected against
Two or more concurrent disk failures within a single RAID group
Storage subsystem failures (HBA, cables, shelf) that lead to two or more concurrent disk failures within a single RAID group
Two or more concurrent disk failures in a single RAID group on both plexes
Required disk resources per RAID group
n data disks + 1 parity disk
2 x (n data disks + 1 parity disk)
Performance cost
None
Low mirroring overhead; can improve performance
Additional cost and complexity
None
SyncMirror license and configuration
RAID0 and SyncMirror
Criteria
RAID0 alone
RAID0 with SyncMirror
Failures protected against
RAID0 does not provide protection against any failures. RAID protection is provided by the RAID implemented on the third-party storage array.
Any combination of array LUN, connectivity, or hardware failures, as long as only one plex is affected
Failures not protected against
RAID0 does not provide protection against any failures. RAID protection is provided by the RAID implemented on the storage array.
Any concurrent failures that affect both plexes.
Required array LUN resources per RAID group
No extra array LUNs required other than n data array LUNs
2 x n data array LUNs
Performance cost
None
Low mirroring overhead; can improve performance
Additional cost and complexity
None
SyncMirror license and configuration

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