How Data ONTAP uses RAID to protect your data and data availability

RAID protects your data and data availability. Understanding how RAID provides this protection can help you administer your storage systems more effectively.

Data ONTAP uses RAID-DP (double-parity) or RAID Level 4 (RAID4) protection to ensure data integrity within a group of disks even if one or two of those disks fail. This is done by using parity disks, which provide redundancy for the data stored in the data disks. If a disk fails (or, for RAID-DP, up to two disks), the RAID subsystem can use the parity disks to reconstruct the data in the drive that failed.

Note: V-Series systems use RAID-DP and RAID4 only for native disk shelves. For third-party storage arrays, the V-Series system stripes across array LUNs using RAID0. A higher level of RAID protection is not needed because the array LUNs are already protected by the redundancy provided by the RAID level on the storage array.

RAID protection levels
Data ONTAP supports two levels of RAID protection, RAID-DP and RAID4. RAID-DP can protect against double-disk failures or failures during reconstruction. RAID4 can protect against single-disk failures. You assign RAID level on a per-aggregate basis.
RAID disk types
Data ONTAP classifies disks as one of four types for RAID: data, hot spare, parity or dparity. The RAID disk type is determined by how RAID is using a disk.
How RAID groups work
The purpose of a RAID group is to provide parity protection from data loss across its included disks.
Protection provided by RAID and SyncMirror
Combining RAID and SyncMirror provides protection against more types of disk failures than using RAID alone.
How Data ONTAP works with hot spares
A hot spare disk is a disk that is assigned to a storage system but is not in use by a RAID group. It does not yet hold data but is ready for use. If a disk failure occurs within a RAID group, Data ONTAP automatically assigns hot spare disks to RAID groups to replace the failed disks.
How Data ONTAP handles a failed disk with an available hot spare
Using an available matching hot spare, Data ONTAP can use RAID to reconstruct the data on the failed disk with no data service interruption.
How Data ONTAP handles a failed disk that has no available hot spare
When a failed disk has no appropriate hot spare available, Data ONTAP puts the affected RAID group into degraded mode indefinitely and the storage system automatically shuts down within a specified time period.
How Data ONTAP handles media errors during reconstruction
By default, if Data ONTAP encounters media errors during a RAID reconstruction, it automatically invokes an advanced mode process (wafliron) that compensates for the media errors and enables Data ONTAP to bypass the errors.
RAID status
You use the aggr status -r command to check the current RAID status and configuration for your aggregates.
How RAID-level disk scrubs verify data integrity
RAID-level scrubbing means checking the disk blocks of all disks in use in aggregates (or in a particular aggregate, plex, or RAID group) for media errors and parity consistency. If Data ONTAP finds media errors or inconsistencies, it uses RAID to reconstruct the data from other disks and rewrites the data.