Re: [Xen-users] xen disk io performance

[Alexandre Chapellon <a.chapellon@xxxxxxxxx>]

Le 28/11/2011 10:36, Niels Dettenbach a écrit :
> Dear Alexandre,
>
> Am Montag, 28. November 2011, 10:03:23 schrieb Alexandre Chapellon:
> > disk =[ 'phy:/dev/xendata/postgresql-syslog:xvdb1:w']
> This is typically correct.
>
> > What can be done to improve disk IO and throuput?
> I did not know much about the NetApp SAN you describe but there are usually
> different possible reasons for performance problems of databases on SAN
> storage.
>
> One of the major bottlenecks i saw often was a very limited transaction rate
> (number of i/o requests) of the SAN (if the SAN cache was filled) even if the
> SAN offers a high bandwidth / throughput when writing large files etc.. What
> kind of RAID configuration do you use within your SAN? Are there other
> applications / users which use the same disk space / disks in parallel? How
> many "free" disk heads are available for your pgsql? What kind of filesystems
> do you use?
the raid is raid_dp, of 13 FC disks (15000rpm) and the LV containing 
postgres data is stripped accross 2 raid group of that kind (26 disks), 
filesystem is ext4.
Am not very familiar with benchmarking tools, for now I just used 
tiobench and monitor tools like iotop or iostats... The thing is am not 
sure how to read the result.
> You may take some tests with storage benchmarking tools to go down to the
> source of the bottleneck (make shure your benachmark fill up / eliminates the
> SAN cache for usuable results).
>
> On the Xen level byself i did not see any further optimization options -
> typical things you have to review for any optimizations are:
>
>   - reduce (swappiness) or disable swapping at all
>   - optimize RAM usage (buffering)
>   - (if possible) realize exclusive access of pgsql to the physical disks
>
> In several cases filesystem behaviours / suboptimal block sizes may interfer
> to performance too.
The thread is now offtopic, but If you don't mind I'd be glad to hear 
about good sources on how to deal with filesystem block siez 
calculation, partition alignement and its implications on LVM...
Regards
> hth
> best regards,
>
>
> Niels.
>
>
>
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Alexandre Chapellon

Ingénierie des systèmes open sources et réseaux.
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