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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 

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.

http://www.coker.com.au/selinux/   My NSA Security Enhanced Linux packages
http://www.coker.com.au/bonnie++/  Bonnie++ hard drive benchmark
http://www.coker.com.au/postal/    Postal SMTP/POP benchmark
http://www.coker.com.au/~russell/  My home page

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