HP Smart Storage Adstrator manual Selecting a RAID method, Alternative fault-tolerance methods

Item	RAID 0	RAID 1+0	RAID 5	RAID 6	RAID 1(0)
				(ADG)	(ADM)
Alternative name	Striping (no	Mirroring	Distributed	Advanced	Advanced
	fault		Data	Data	Data
	tolerance)		Guarding	Guarding	Mirroring
Formula for number of drives usable	n	n/2	n-1	n-2	n/3
for data (n = total number of drives in
array)
Percentage of drive space usable*	100%	50%	67% to 93%	50% to 96%	33%
Minimum number of physical drives	1	2	3	4	3
Tolerates failure of one physical drive	No	Yes	Yes	Yes	Yes
Tolerates simultaneous failure of	No	Only if no	No	Yes	Only if no
more than one physical drive		two failed			three drives
		drives are in			are in the
		the same			same mirror
		mirrored			group**
		pair
Read performance	High	High	High	High	High
Write performance	High	Medium	Low	Low	Medium
Relative cost	Low	High	Medium	Medium	Very high

*Values for the percentage of drive space usable are calculated with these assumptions: (1) all physical drives in the array have the same capacity; (2) online spares are not used; (3) no more than 14 physical drives are used per array for RAID 5; and (4) no more than 56 drives are used with RAID 6 (ADG).

**Mirror groups include the physical drives in each mirror.

Selecting a RAID method

Not all controllers support all RAID levels. To determine the RAID capabilities of your controller, see the model-specific information for your controller on the HP website (http://www.hp.com/products/smartarray).

Most important criterion	Also important	Suggested RAID level
Fault tolerance	Cost effectiveness	RAID 6
	I/O performance	RAID 10 (ADM), RAID 1+0, RAID 50, RAID 60
Cost effectiveness	Fault tolerance	RAID 6
	I/O performance	RAID 5 (RAID 0 if fault tolerance is not required)
I/O performance	Cost effectiveness	RAID 5 (RAID 0 if fault tolerance is not required)
	Fault tolerance	RAID 10 (ADM), RAID 1+0, RAID 50, RAID 60

Alternative fault-tolerance methods

Your operating system may also support software-based RAID or controller duplexing.

•Software-based RAID resembles hardware-based RAID, except that the operating system works with logical drives as if they were physical drives. To protect against data loss caused by physical drive failure, each logical drive must be in a different array from the others.

•Controller duplexing uses two identical controllers with independent, identical sets of drives containing identical data. In the unlikely event of a controller failure, the remaining controller and drives will service all requests.

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