SCSI Solutions White Paper - HP-UX

The A9890A RAID card is a PCI-X 2-port Ultra320 SCSI RAID Controller which provides

Advanced Data Guarding (RAID ADG), leading performance, 256MB Double Data Rate (DDR)

battery-backed write cache architecture, and a new RAID engine. The A9891A is very similar

to the A9890A card, the difference being that the A9891A has two additional SCSI ports for a

total of four. The A7143A is a PCI 4-port Ultra160 SCSI RAID Controller with similar RAID and

cache features.

For supported configurations of OS versions, platforms, and devices with these RAID controllers

see http://docs.hp.com/en/SM-20050510/raidcombinedsupportmatrix.htm.

All three cards support the following RAID levels, from lowest to highest in fault tolerance:

§ RAID 0 – Striping (no fault tolerance)

§ RAID 1 – Mirroring

§ RAID 1+0 – Mirroring and Striping

§ RAID 5 – Distributed Data Guarding (Striping with Parity)

§ RAID ADG – Advanced Data Guarding (Striping with Double Parity)

RAID 0 enhances performance with data striping, but provides no data redundancy to protect

against data loss when a physical disk fails. RAID 1 mirrors the data onto two disks without

striping.

RAID 1+0 provides both the redundancy of mirroring and the performance benefits of striping,

with the disks mirrored in pairs and the data striped across the mirrored pairs. This allows for

the failure of multiple disks in the array without incurring data loss, as long as no two failed

disks belong to the same mirrored pair. RAID 1+0 has the highest read and write performance

of any of the fault-tolerant RAID levels, but is more costly per unit of storage, with only one-half

of the disk space being available for non-redundant data (usable capacity).

RAID 5, Distributed Data Guarding, uses striping with parity, with the parity data distributed

across the physical disks. When a physical disk fails, the data that was on the failed disk can

be calculated from the data blocks on the remaining physical disks. RAID 5 has lower write

performance than RAID 1+0, but has more usable disk capacity: in an array composed of n

physical disks, it has a usable capacity of (n-1)/n, which calculate to a low of 67% in a 3-disk

array up to 93% in a 14-disk array. RAID 5 is recommended for up to 14 physical disks.

HP’s RAID ADG, Advanced Data Guarding, provides the highest level of fault tolerance, by

generating parity data ala RAID 5, but increasing to two sets of parity data per data stripe. As

a result, it can withstand two simultaneous drive failures without downtime or data loss. It also

maintains a very good usable capacity, (n-2)/n, which is close to RAID 5 and higher than the

other fault-tolerant levels. The only significant disadvantage of RAID ADG is a relatively low

write performance (lower than RAID 5), due to the generation of two sets of parity data.

Note: The Smart Array SCSI RAID cards are not supported in multi-initiator configurations.

For additional information on the SCSI RAID products, go to