RAID5 question

[Over Clocker]

Hi guys,

Quick question regarding raid 5:

Would it be better to use 4x 320gb drives, or 3x 500gb drives?

How do you calculate total space each would give? Is there a benefit to using 4 drives in a raid 5 array?

Cheers,

Mal

 

[NickK]

The number of spindles increases speed (more read/write heads and drive buffer/caches) at the cost of some latency for 'dumb' RAID5 controllers.

a lot of RAIDs will drive the full disc so go for the largest drive or you'll be stuck with the smaller drives.
Also be careful of changes in the drive geometry as this may render new drives (with perhaps different geometry) incompatible with the current array set. This can occur with new versions of the same drive as a mate found out.

RAID5 is easy - although the parity is distributed over all drives, the space for parity is always the same and all drives are used for data/parity.
available unformatted drive space = sum of drivespace from (drives-1)
parity will take one drive in 'space'.

3x500 = 1TB of unformatted space
4x320 = 960GB of unformatted space

Adding an additional drive to a RAID5 array does not increase it's resilience. You still have only one simultaneous drive failure possible.

RAID6 uses two discs worth of parity (available = drives-2 instead) and can withstand two drives failing simultaneously.

If you're doing high speed database work then I would run RAID1+0 or 0+1 instead (mirrored RAID0 or RAID0 of mirrored drivers).

So for domestic use large drives.