Protection provided by RAID and SyncMirror

Combining RAID and SyncMirror provides protection against more types of drive failures than using RAID alone.

You can use RAID in combination with the SyncMirror functionality, which also offers protection against data loss due to drive 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 drive failure in one plex is repaired by the undamaged data in the other plex.

For more information about SyncMirror, see the Data ONTAP Data Protection Online Backup and Recovery Guide for 7-Mode.

The following tables show 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-drive failure

•	Double-drive failure within a single RAID group

•	Multiple-drive failures, as long as no more than two drives 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 drive failures within a single RAID group

•	Storage subsystem failures (HBA, cables, shelf) that lead to three or more concurrent drive failures within a single RAID group

•	Three or more concurrent drive failures in a single RAID group on both plexes

Required resources per RAID group

n data drives + 2 parity disks

2 x (n data drives + 2 parity drives)

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 drive failures within a single RAID group

•	Storage subsystem failures (HBA, cables, shelf) that lead to two or more concurrent drive failures within a single RAID group

•	Two or more concurrent drive failures in a single RAID group on both plexes

Required resources per RAID group

n data drives + 1 parity drive

2 x (n data drives + 1 parity drive)

Performance cost

None

Low mirroring overhead; can improve performance

Additional cost and complexity

None

SyncMirror configuration and extra storage requirement

RAID0 and SyncMirror
Criteria	RAID0 alone	RAID0 with SyncMirror
Failures protected against	No protection against any failures RAID protection is provided by the RAID implemented on the storage array.	Any combination of array LUN, connectivity, or hardware failures, as long as only one plex is affected
Failures not protected against	No 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 configuration and extra storage requirement