[sclug] "Adding RAID" - what does that mean nowadays? I thought PCI bus was really slow

Will Dickson wrd at glaurung.demon.co.uk
Tue Mar 18 20:17:10 UTC 2008

M.Blackmore wrote:
> Someone in an earlier thread mentioned adding RAID as being a major
> value-added increment he gave to computers he made up for clients, as
> this speeded up the disk access bottleneck.

Pace the earlier thread (which I don't remember, and which I'm too lazy 
to look for), I looked into this idea quite recently; I needed a bigger 
HDD, and the new ones were so huge and so cheap that it looked like an 
interesting proposition.

RAID 0: Risky - having two disks in a unit halves the MTBF of the unit, 
since if you lose one disk you lose the whole array. I decided this 
looked like a Bad Idea.

RAID 1: This looked quite interesting: you get the improved resilience 
which comes from having redundant copies, and although write performance 
may be somewhat worse, in theory you get double the read performance, by 
taking half the file from each disk in the mirror. In practice, the read 
performance gains were much less in the tests I read about: it was a bit 
variable, but overall not much better than about 10%.

RAID 5 et seq: too many disks.

Given the extra complications and portability risks (if you're using the 
RAID controller on the mobo, and the mobo dies, will you be able to read 
the RAID back with the new mobo? If you're using OS software RAID, will 
you be able to dual-boot?) I decided that it just wasn't worth it.

As far as I could gather, the consensus among those who know what 
they're talking about (unlike me :-) is that the main point of a RAID is 
to buy you an hour or two in which to yank out the dead disk and swap in 
a new one, without having to take the affected server offline, or (much 
worse) having to rebuild it. If you're in a biggish datacentre, then 
losing a disk is going to be a regular occurrence, simply because you're 
herding such a lot of them. If you're not, then losing a disk is a rare 
event, and the only point of having a RAID is the amusement value and / 
or bragging rights.

In particular, RAID is no substitute for proper backups.



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