Understanding RAID groups

About RAID groups in Data ONTAP

A volume consists of disks assembled into one or multiple RAID groups. A RAID group consists of one or more data disks, across which client data is striped and stored, plus one or two parity disks that ensure data recoverability even if as many as two disks within the RAID group fail. The purpose of a RAID group is to provide parity protection of data across its included disks from loss due to single or double disk failure within that group.

Types of RAID protection

Data ONTAP supports two types of RAID protection, RAID 4 or RAID DP, which you can assign on a per-volume basis.

RAID 4 protection

RAID 4 provides single-parity disk protection against single disk failure within a RAID group. The minimum number of disks in a RAID 4 group is two: at least one data disk and one parity disk. In case of single disk failure, Data ONTAP replaces the failed disk in the RAID group with a spare disk and uses the parity data to reconstruct the failed disk's data on the replacement disk. The following figure diagrams a volume configured for RAID 4 protection.

RAID DP protection

RAID DP provides double-parity disk protection against single or double disk failure within a RAID group. The minimum number of disks in a RAID DP group is three: at least one data disk, one regular parity disk, and one double-parity (or dParity) disk. (A three-disk root volume, for example, could be parity-protected even if one data and one parity disk failed).

In case of single disk failure, Data ONTAP replaces the failed disk in the RAID group with a spare disk and uses the parity data to reconstruct the data of the failed disk on the replacement disk. In case of double disk failure, Data ONTAP replaces the failed disks in the RAID group with two spare disks and uses the double-parity data to reconstruct the data of the failed disks on the replacement disks. The following figure diagrams a volume configured for RAID DP protection.

How Data ONTAP creates RAID groups automatically

When you create a filer volume or add disks to a filer volume, Data ONTAP creates new RAID groups as each RAID group is filled with its maximum number of disks. Within each volume, RAID groups are named rg0, rg1, rg2, and so on in order of their creation. The last RAID group formed might contain fewer disks than are specified for the volume's RAID group size. In that case, any disks added to the volume are also added to the last RAID group until the specified RAID group size is reached.

If a volume is configured for RAID 4 protection, Data ONTAP assigns the role of parity disk to the largest disk in each RAID group.
Note
If an existing RAID 4 group is assigned an additional disk that is larger than the group's existing parity disk, then Data ONTAP reassigns the new disk as parity disk for that RAID group.
If a volume is configured for RAID DP protection, Data ONTAP assigns the role of dParity disk and regular parity disk to the largest disk and the second-larger disk in the RAID group.
Note
If an existing RAID DP group is assigned an additional disk that is larger than the group's existing dParity disk, then Data ONTAP reassigns the new disk as the regular parity disk for that RAID group but restricts its capacity to be no greater than that of the existing dParity disk.

Hot spare disks

A hot spare disk is a filer disk that has not been assigned to a RAID group. It does not yet hold data but is ready for use. In event of disk failure within a RAID group, Data ONTAP automatically assigns hot spare disks to RAID groups to replace the failed disks. Hot spare disks do not have to be in the same disk shelf as other disks of a RAID group to be available to a RAID group.

Hot spare disk size recommendations

Network Appliance recommends keeping at least one hot spare disk for each disk size and disk type installed in your filer. This allows the filer to use a disk of the same size and type as a failed disk when reconstructing a failed disk. If a disk fails and a hot spare disk of the same size is not available, the filer uses a spare disk of the next available size up. See Disk failure with a hot spare disk for more information.

Note
If no spare disks exist in a filer, Data ONTAP can continue to function in degraded mode. Data ONTAP supports degraded mode in case of single disk failure for volumes configured with RAID 4 protection and in case of single or double disk failure in volumes configured for RAID DP protection. For details see Disk failure without a hot spare disk.

Maximum number of RAID groups

Data ONTAP enables you to support up to 400 RAID groups per filer or cluster. When configuring your volumes, keep in mind that each volume requires at least one RAID group and that the total of all RAID groups in a filer cannot exceed 400.

RAID 4, RAID DP, and SyncMirror

RAID 4 or RAID DP 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 mirrored plexes of the same volume. Any data loss due to disk failure in one plex is repaired by the undamaged data in the opposite plex. The advantages and disadvantages of using RAID 4 or RAID DP, with and without the SyncMirror feature, are listed in the following table.

Factor affected by RAID type	RAID 4	RAID DP	RAID 4 with SyncMirror	RAID DP with SyncMirror
The kinds of failures that RAID and SyncMirror protect against	Single disk failure within one or multiple RAID groups	Single or double disk failure within one or multiple RAID groups	Single, double, or greater disk failure within one or multiple RAID groups across one sync-mirrored plex. (See Advantages of RAID 4 with SyncMirror.) Storage subsystem failures (HBA, cables, shelf) on the filer	Single, double, or greater disk failure within one or multiple RAID groups across both sync-mirrored plexes. (See Advantages of RAID DP with SyncMirror.) Storage subsystem failures (HBA, cables, shelf) on the filer
Required disk resources per RAID group	n data disks + 1 parity disk	n data disks + 2 parity disks	2 x (n data disks + 1 parity disk)	2 x (n data disks + 2 parity disks)
The performance cost	None	Almost none	Mirroring overhead	Mirroring overhead
Additional cost and complexity	None	None	SyncMirror license and configuration Possible cluster license and configuration	SyncMirror license and configuration Possible cluster license and configuration

Advantages of RAID 4 with SyncMirror

On SyncMirror-replicated volumes using RAID 4, any combination of multiple disk failures within single RAID groups in one plex is restorable, as long as multiple disk failures are not concurrently occurring in the same RAID groups in the opposite plex of the mirrored volume. Between two mirrored RAID groups, a concurrent failure of any three disks is tolerable.

Advantages of RAID DP with SyncMirror

On SyncMirror-replicated volumes using RAID DP, any combination of multiple disk failures within single RAID groups in one plex is restorable, as long as concurrent failures of greater than two disks are not occurring in the same RAID groups in the opposite plex of the mirrored volume. Between mirrored RAID groups, concurrent failure of any 5 disks is tolerable.

For more SyncMirror information

For more information on the Data ONTAP SyncMirror feature, see the Data ONTAP System Administration Data Protection Guide.

Larger versus smaller RAID groups

Configuring an optimum RAID group size for a volume requires a trade-off of factors. You must decide which factor--speed of recovery, assurance against data loss, or maximizing data storage space-- is most important for the volume that you are configuring. For a listing of default and maximum RAID group sizes, see Maximum and default RAID group sizes.

Advantages of large RAID groups

Large RAID group configurations offer the following advantages:

More data drives available. A volume configured into a few large RAID groups requires fewer drives reserved for parity than that same volume configured into many small RAID groups.
Better system performance. Read/write operations are usually faster over large RAID groups than over smaller RAID groups.

Advantages of small RAID groups

Small RAID group configurations offer the following advantages:

Shorter disk reconstruction times. In case of disk failure within a small RAID group, data reconstruction time is usually shorter than it would be within a large RAID group.
Decreased risk of data loss due to multiple disk failures. The probability of data loss through double disk failure within a RAID 4 group or through triple disk failure within a RAID DP group is lower within a small RAID group than in a large RAID group.