Re: [Xen-users] xen disk io performance

[Niels Dettenbach <nd@xxxxxxxxxxxx>]

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?

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.


hth
best regards,


Niels.

-- 
---
Niels Dettenbach
Syndicat IT&Internet
http://www.syndicat.com/

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