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3,196 Views 2 Replies Last post: Oct 20, 2010 1:47 PM by florent42 RSS
florent42 Lurker
florent42
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Oct 20, 2010
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Oct 20, 2010 10:15 AM

Bad performance (huge write latency) on PERC H200 (DELL - mptsas)

Hi.

 

I have two servers. One DELL R510 with PERC6/i (megaraid_sas driver), and one DELL R210 with PERC H200 (mptsas driver). They're both configured as RAID1 (mirroring).

 

On the R510, guests have great performance. ESXi reports a datastore write latency between 0 and 2ms. Wonderful.

 

However, on the R210, guests are incredibly slow, due to I/O waiting. "top" shows 20% of "wa" (I/O wait), and guest load is usually around 0.20 (while doing nothing!). Maximum write speed is around 6 Mb / s. Not really usable.

 

ESXi reports datastore write latency between 200ms and 500ms. That explains bad guests performance. Is this a known issue with this hardware ?

RAID status is "OPTIMAL", so the latency does not come from a recovery.

 

What can I do ? Are there some hidden configurations or actions that could be done via ssh or through the vSphere Client to get decent performance ?

 

Thanks for your help.

 

Regards,

Florent

 

Message was edited by: florent42 - added OPTIMAL state

Tags: performance, storage, dell, datastore, esxi, perc, latency, vsphere, vsphere_4, esxi_4, mptsas, h200
a.p. Guru User Moderators vExpert
a.p.
11,726 posts since
May 18, 2007
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1. Oct 20, 2010 10:33 AM in response to: florent42
Re: Bad performance (huge write latency) on PERC H200 (DELL - mptsas)

Welcome to the Community,

 

from what I read about the RAID controllers you mention, The PERC 6/i has 256 MB BBU cache and the H200 does not have cache (at least no write cache). Therefore it operates in write-through mode what most likely causes the latency you see.

The only solution I see is getting another RAID controller with BBU which can operate in write-back mode.

 

André

 

EDIT: Link to the DELL RAID controlles

florent42 Lurker
florent42
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Oct 20, 2010
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2. Oct 20, 2010 1:47 PM in response to: a.p.
Re: Bad performance (huge write latency) on PERC H200 (DELL - mptsas)

I see. Furthermore, the H200 doc (http://support.dell.com/support/edocs/storage/Storlink/H200/en/UG/HTML/features.htm#wp1062398) says :

 

 

On a PERC H200 card, caching is forced to be disabled for all physical disks configured into a virtual disk, regardless of the drive type and default drive settings.

 

I understand there is RAID controller cache (not present on H200) + physical disk cache (disabled on H200), which explains the bad performance.

 

Thank you, I will look into a PERC 6/i controller.

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