Advanced Data Guarding User Guide

E-2 Advanced Data Guarding for the Smart Array 5300 Controller Reference Guide

Compaq Confidential – Need to Know Required

Writer: John Turner Project: Advanced Data Guarding for the Smart Array 5300 Controller Reference Guide Comments: final draft

Part Number: 188270-xx1 File Name: i-appE Questions and Answers.doc Last Saved On: 12/11/00 12:08 PM

Q: What is the difference between RAID ADG and RAID 0+1?

A: RAID ADG uses the capacity of only two hard drives for fault tolerance,

regardless of the size of the RAID volume. This means a lower cost of

implementation for six or more drives.

RAID ADG also provides a higher level of fault tolerance, since the

mathematical probability of suffering a second drive failure in the same

mirror set as an already-failed drive (in RAID 0+1) is higher than the

probability of a third drive failure in RAID ADG. However, RAID 0+1 is

the highest-performance RAID level.

Q: What is the difference between RAID ADG and RAID 5?

A: RAID ADG allows two drives to fail simultaneously without downtime or

data loss. (Data is also preserved if one drive fails and another simply has

a few bad sectors.) However, RAID 5 only allows one drive to fail. RAID

ADG thus provides a much higher level of fault tolerance than RAID 5.

Due to this higher level of protection, users can set up larger RAID

volumes, spanning up to 56 physical drives. With RAID 5, the maximum

recommended number of drives per volume is only 14.

RAID 5 and RAID ADG have equal performance when reading data, but

RAID ADG is slower when writing data due to the extra parity set that it

uses. The exact difference in performance will vary from one situation to

another.

Q: What products support RAID ADG?

A: RAID ADG is available with controller model SA-5304/128. It is also

supplied with the 128-MB cache module available as an option for the

SA-5302/32 and SA-5302/64 controllers.

Q: Does RAID ADG support online spares?

A: Yes. Customers can incorporate online spares for added data protection.

Q: How is RAID ADG activated?

A: Both a hardware key and a firmware upgrade are required. If the hardware

key (RAID ADG Enabler Module) is missing or faulty, the write cache is

disabled and the user cannot configure any further RAID ADG volumes.

Customers who buy SA-5304/128 controllers will be able to download the

firmware upgrade to activate RAID ADG from the Compaq website at