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Re: extremely slow write performance plaintext

To: xfs@xxxxxxxxxxx
Subject: Re: extremely slow write performance plaintext
From: Cory Coager <ccoager@xxxxxxxxxxxxxxx>
Date: Tue, 18 Jan 2011 09:16:47 -0500
Banner: Set
Cc: Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>
In-reply-to: <4D30C7F3.3040105@xxxxxxxxxxxxxxxxx>
Organization: Davis Vision
References: <3205_1294953756_4D2F6D1C_3205_1943_1_4D2F6D1C.2060409@xxxxxxxxxxxxxxx> <20110113233527.6dca104d@xxxxxxxxxxxxxx> <18993_1294964274_4D2F9632_18993_1387_1_F79CF9ADB27B2646B59221B7355263830CC143B1@xxxxxxxxxxxxxxxxxxxxxxxx> <4D30A945.4060000@xxxxxxxxxxxxxxxxx> <27616_1295038110_4D30B69E_27616_233_1_4D30B69E.4020506@xxxxxxxxxxxxxxx> <4D30C7F3.3040105@xxxxxxxxxxxxxxxxx>
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On 01/14/2011 05:02 PM, Stan Hoeppner wrote:
The controller should do this automatically.  You'll have to check the docs to
verify.  This is to safeguard data.  The BBWC protects unwritten data in the
controller cache only, not the drives' caches.  It won't negatively affect
performance if the drives' caches are enabled.  On the contrary, it would
probably increase performance a bit.  It's simply less safe having them enabled
in the event of a crash.

After rereading your original post I don't think there's any issue here anyway.
  You stated you have 24 drives in 2 arrays (although you didn't state if all 
disks are on one P600 or two).

Just one P600.

This was the important part I was looking for.  It's apparently not a cache
issue then, unless the utility is lying or querying the wrong controller or

Nothing relevant in dmesg or any other logs?  No errors of any kind?  Does
iostat reveal anything even slightly odd?
Nothing interesting in dmesg. iostat looks pretty dead on average. During the dd write its doing about ~7tps according to iostat.
I also just noticed you're testing writes with a 1k block size.  That seems
awefully small.  Does the write throughput increase any when you test with a
4k/8k/16k block size?
Yes, it the throughput does increase with larger block sizes. I was able to get ~13MB/s with 16k block size, still terrible however.
BTW, this is an old machine.  PCI-X is dead.  Did this slow write trouble just
start recently?  What has changed since it previously worked fine?
The array is new and newly implemented.
You're making it very difficult to assist you by not providing basic
troubleshooting information.  I.e.

What has changed since the system functioned properly?
When did it change?
Did it ever work properly?

God I hate pulling teeth... :)
No, it has never worked properly. Also, I want to stress that I am only having performance issues with one logical volume, the others seem fine.

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