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Date: Fri, 23 Nov 2007 11:54:00 +0100
From: Kris Kennaway <kris@FreeBSD.org>
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To: Alexey Popov <lol@chistydom.ru>
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Cc: freebsd-hackers@freebsd.org, Panagiotis Christias <christias@gmail.com>,
	freebsd-stable@freebsd.org
Subject: Re: amrd disk performance drop after running under high load
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Alexey Popov wrote:
> Kris Kennaway wrote:
> 
>>>>> what is your RAID controller configuration (read ahead/cache/write
>>>>> policy)? I have seen weird/bogus numbers (~100% busy) reported by
>>>>> systat -v when read ahead was enabled on LSI/amr controllers.
>>>> I tried to run with disabled Read-ahead, but it didn't help.
>>> I just ran into this myself, and apparently it can be caused by 
>>> "Patrol Reads" where the adapter periodically scans the disks to look 
>>> for media errors.  You can turn this off using -stopPR with the 
>>> megarc gg port.
>> Oops, -disPR is the correct command to disable, -stopPR just halts a 
>> PR event in progress.
> Wow! Really disabling Patrol Reads solves the problem. Thank you!
> 
> I have many amrd's and all of them appear to have Patrol Reads enabled 
> by default. But the problem happenes only on three of them. Is this a 
> hardware problem?

I am not sure, maybe for some reason the patrol reads are not 
interfering with other disk I/O so much (e.g. the hardware prioritises 
them differently or something).

Anyway, glad to hear it was resolved.

Kris