On 24 Apr 2009 at 16:06, Konrad Rzeszutek wrote:

> On Fri, Apr 24, 2009 at 02:14:43PM -0400, Donald Williams wrote:
> > Have you tried increasing the disk readahead value?
> > #blockdev --setra X /dev/<multipath device>
> > 
> >  The default is 256.    Use --getra to see current setting.
> > 
> >  Setting it too high will probably hurt your database performance.  Since
> > databases tend to be random, not sequential.
> I would think that the databases would open the disks with O_DIRECT
> bypassing the block cache (And hence the disk readahead value isn't used
> at all).


first a silly question: Shouldn't the read-ahead on the server be as least as 
hight as the setting on the client to provide any benefit?

And two interesting numbers: On one of our busy databases the the read:write 
is about 10:1 and the tables are severely fragmented
On our Linux servers using iSCSI the read:write ratio is about 1:10 because the 
machines have several GIGs of RAM and the disk caching is very efficient. So 
machine just has to send out the writes...


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