[mythtv-users] Wishlist feature for adding a hard drive

[Stephen Boddy]

On Wednesday 04 October 2006 20:00, Andrew Davis wrote:
> A RAID5, for example, will give you a bit better
> performance simply since the writes are spread over multiple disks.

This will only apply if you have a good _hardware_ RAID5 capable card.

If you're using software RAID (md) then your writes will take a hit with 
RAID5. To write data to a disk the following would have to be done at 
minimum. Read at least the old data from the disk to be written to, and the 
disk the old parity is on. Using these and the new data the CPU has to 
calculate the new parity and then write to two disks (new data + new parity).

OD xor OP xor ND = NP
Write ND and NP

This is not a correct description of SW RAID, Just an illustration that RAID5 
is not benificial for writes.

RAID5 will benefit reads though.
-- 
Steve Boddy