Re: I/O performance issues on 2.4.23 SMP system
On Fri, 30 Jan 2004 01:02, Jeff S Wheeler <jsw@five-elements.com> wrote:
> I don't know anything about thos 2.4.23 I/O problem, but I will tell you
> that RAID 5 is not the way to go for big SQL performance. In a RAID 5
> array, all the heads must move for every operation. You already spent a
> lot of money on that server. I suggest you buy more disks for RAID 10.
Any decent RAID-5 implementation will have a non-volatile write-back cache.
This will hugely increase performance as it allows the possibility of
combining writes. NB This is something that Linux software RAID lacks
support for.
Moving all heads is not required for every operation. Reading from all disks
is not required for a read unless an entire line is to be brought in, last
time I did read benchmarks it seemed that this wasn't being done on Mylex
RAID controllers or Sun Metadisk (never done any real tests on Linux software
RAID-5).
Reading from all disks is not necessarily required for a one-block write
either. Reading the block that is to be written and the parity block is
enough. New parity block will be old_block ^ old_parity ^ new_block. Doing
reads from and writes to two disks should be significantly faster than reads
from all disks and writes to two.
The benchmark results Craig Sanders posted when comparing RAID-5 and RAID-10
were surprising, RAID-5 won many of the test scenarios! I recall that Craig
posted the results to this list, a google search should return them.
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