9.1 Understanding Software RAID Devices

A software RAID is a configuration for storage devices that emulates a hardware RAID device. A software RAID combines partitioned space from multiple physical devices into a single virtual device that you manage like any device. Each member device contributes an equal amount of space to the RAID. You can create partitions, pools, and volumes on a RAID device, just as you would with any physical storage device. Unlike hardware RAID devices, software RAIDs use standard host adapters and do not require any special RAID hardware.

The following table describes the software RAID devices supported by NSS:

Table 9-1 RAID Characteristics

Type of RAID

Number of Segments

Purpose

Advantages

Disadvantages

RAID 0

2 to 14

Data striping

Improves I/O performance for both reads and writes, which occur concurrently in parallel to its member devices.

Does not provide data redundancy for data fault tolerance.

If a single disk fails, the data cannot be recovered. You must re-create the RAID 0 and restore its volumes from a backup copy before you can use it again.

RAID 1

2 to 4

Data mirroring

Provides full data redundancy for failover and instant recovery.

Improves read performance.

Equivalent write performance is possible with a duplex connection, which provides a separate channel for each member disk.

To achieve the best I/O performance, it requires separate channels for each member disk; otherwise, write performance decreases slightly.

Each mirror must be on a separate device; it can share no disks in common.

Can be a member of only one pool.

RAID 5

3 to 14

Data striping with parity

Provides limited data recovery for one member disk at a time. If a single drive in the RAID fails, its volumes and pools remain active, but with degraded performance because the RAID must use parity to reconstruct the missing data. You must remove the failed segment, replace the disk, add the new segment, and restripe the data to reconstruct the data on the replacement drive.

Improves read performance if all drives are present and working properly. If a drive fails, read performance is reduced because of parity reads and data reconstruction.

Read responses are the same only if data happens to be in cache when called; otherwise it is slightly reduced for parity checking.

I/O performance for writes is reduced because it takes time to calculate and write parity to disk. The more writes to the drive, the greater is the burden to CPU.

If multiple disks fail, the data cannot be recovered. You must re-create the RAID 5 and restore its volumes from a backup copy before you can use it again.

RAID 10

2 to 4 RAID 0 devices

Mirroring RAID 0 devices

Provides full data redundancy for failover and instant recovery.

Improves I/O performance for both reads and writes, but is slower than an unmirrored RAID 0 device.

Requires separate channels for each member disk to achieve best I/O performance.

RAID 0 devices that you mirror can share no disks in common.

If a single disk fails, you must recreate the RAID 0 and remirror the entire device. The data is restored through mirroring.

RAID 15

2 to 4 RAID 5 devices

Mirroring RAID 5 devices

Provides full data redundancy for failover and instant recovery.

If a single data disk fails, the RAID 5 device remains up and mirrored. Its performance is degraded until you replace the failed disk.

It can handle multiple disk failures, depending on the number of failures and which disks fail.

Requires separate channels for each member disk to achieve best I/O performance.

RAID 5 devices that you mirror can share no disks in common.

If multiple data disks fail concurrently on the same segment or if the parity disk fails, you must remove the damaged segment from the mirror, re-create the RAID 5, and then mirror the RAID 5. The data synchronizes with the mirrors.