1 processor vs. 2

[Scott W]

Stefan Cars wrote:
> Ok. In this case the costs isn't really a problem, so both read and
> write will be faster with two disks in a RAID1 vs. three disks in a RAID
> 5 ? I've read that RAID5 would be faster in read ?

Short answer- it depends.  Bear in mind that there are some controllers 
that will do RAID-0(striping) or RAID-1(mirroring)- these generally come 
without cache on the controller, which is a huge hit for most cases, 
especially if you enable write-back cache, which will return from the 
write operation(s) once it's commited to _cache_ and not nescessarily to 
disk.

If you're talking similar number of disks versus previous example of 
simple 2 disk mirror versus 10-disk RAID-5, reads will become 
signficantly faster due to the increased number of spindles in the 
array.  I've done a fair amount of testing here- stripe size and cache 
size can be important, but disk I/O is ultimately limited by a large 
factor to the number of drive spindles in use.

In theory, and for a low amount of writes, the overhead for 
RAID-1/mirroring is relatively low, but may increase under high load 
with a large amount of data being written.

For the same number of _useable_ disks, RAID-5 is slower due to having 
to calculate parity (read useable disks as using the same hard drives, 
same capacity and specs, to wind up at the same amount of useable storage).

Scott



> / Stefan
> 
> On Wed, 3 Mar 2004, Matthew Seaman wrote:
> 
> 
>>On Wed, Mar 03, 2004 at 10:26:43PM +0100, Stefan Cars wrote:
>>
>>>Okey, but if you would compare RAID-1 on two disks compared to RAID-5 on
>>>three disks then ? What would be the faster ?
>>
>>RAID1 is going to be faster, both reading and writing, but it will
>>take a lot more raw disk space to provide the required usable space.
>>
>>	Cheers,
>>
>>	Matthew
>>
>>--
>>Dr Matthew J Seaman MA, D.Phil.                       26 The Paddocks
>>                                                      Savill Way
>>PGP: http://www.infracaninophile.co.uk/pgpkey         Marlow
>>Tel: +44 1628 476614                                  Bucks., SL7 1TH UK
>>
> 
> 
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> Stefan Cars
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