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Date: Fri, 25 Jul 2008 11:58:53 +0200
From: Kris Kennaway <kris@FreeBSD.org>
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To: Jeremy Chadwick <koitsu@FreeBSD.org>
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Cc: FreeBSD Stable <freebsd-stable@freebsd.org>,
	Claus Guttesen <kometen@gmail.com>
Subject: Re: zfs, raidz, spare and jbod
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Jeremy Chadwick wrote:
> On Fri, Jul 25, 2008 at 09:46:34AM +0200, Claus Guttesen wrote:
>> Hi.
>>
>> I installed FreeBSD 7 a few days ago and upgraded to the latest stable
>> release using GENERIC kernel. I also added these entries to
>> /boot/loader.conf:
>>
>> vm.kmem_size="1536M"
>> vm.kmem_size_max="1536M"
>> vfs.zfs.prefetch_disable=1
>>
>> Initially prefetch was enabled and I would experience hangs but after
>> disabling prefetch copying large amounts of data would go along
>> without problems. To see if FreeBSD 8 (current) had better (copy)
>> performance I upgraded to current as of yesterday. After upgrading and
>> rebooting the server responded fine.
> 
> With regards to RELENG_7, I completely agree with disabling prefetch.
> The overall performance (of the system and disk I/O) appears signicantly
> "smoother", e.g. less hard lock-ups and stalls, is better when prefetch
> is disabled.

FYI I do not get "lock-ups" when running with prefetch.  It is supposed 
to just affect performance, i.e. if you have few disks or they have low 
bandwidth or high seek times (e.g. crappy ATA) then it can saturate them 
and you will have poor response times.  However if your hardware is more 
capable then it is a performance optimization.

Someone needs to obtain the usual debugging information.

Kris