On Wed, 3 Jan 2001, I (Zero Sum) wrote:
I know it is a bit off topic and if it has been discussed to death
before, I apologise. But for the lfe of me I can't see why anyone would
use RAID as other than an acadaemic exercise.
On Wednesday 03 January 2001 13:40, Jim King wrote in reponse:
Economics. If performance isn't a top priority and I need some fault
tolerance I'd prefer to use RAID 5 and make more efficient use of the disk
space.
On Wednesday 03 January 2001 13:47, Greg Lehey wrote in reponse:
There are many reasons for using RAID. If you're looking for good
read/write performance, you won't use RAID-5. But if you have a web
server, for example, where 99% of all accesses are reads, then RAID-5
is quite a good choice. I do tend to agree that a lot of people use
RAID-5 where RAID-1 would be a better choice.
On Wednesday 03 January 2001 13:49, Tom wrote in reponse:
But you don't use RAID 5 for performance. RAID is about reliablity. If
you want reliability and performance, RAID10 wins. If you are operating
on a budget use RAID5, or if you are putting toghether a lot of storage,
RAID50.
Jim and Tom mention the factor of expense. In Australia, I think we pay
about five times the price that Americans seem to (this is not a
price/currency conversion) but I still can't take that factor seriously.
Disk space is *cheap*. As Tom says, for reliability and performance
RAID1/0 wins. Using RAID 5 is "spoiling the ship for a pennyworth of tar",
or so it seems to me.
Hardware RAID is expensive. If you make that investment, the price of more
disk should not be important.
Software RAID gets slower the more complex it is. RAID 5 is considerably
more complex than 0/1. Wouldn't you be better off spending money on a less
capable processor and more disk? I haven't done any measurements here. I
am only going from (limited) experience.
But I can't see 'expense' as a justifying factor here. Not in a
professional arena.
Greg mentioned a situation (web server) where there are many reads (99%).
I think that for RAID 5 to help matters much, you would have to have a very
high hit rate on an immense number of different static pages. The read
performance of 5 vs 1/0 is there, but it is marginal. Whether it gives you
any improvement or not (how much, if any) is very dependant on what is
going on in bufferspace.
Thanks for your responses, but I still can't see a good argument for RAID5.
Hope I haven't trod on too many toes.
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