ZFS and mem management

[Pavlo]

--- Original message ---
From: "George Kontostanos" <gkontos.mail at gmail.com>
To: "Pavlo" <devgs at ukr.net>
Date: 15 February 2012, 17:39:46
Subject: Re: ZFS and mem management



>

2012/2/15 Pavlo <devgs at ukr.net>:
>
>
> 2012/2/15 Pavlo <devgs at ukr.net>:
>>
>>
>>
>>>On 15/02/2012 13:39, Pavlo wrote:
>>>>
>>
>>>> >> >> Unfortunately we can't afford disabling prefetch. It is too much
>>>> >> >> of an>> overhead.>> >> Also I made some tests. I have process that maps file
>>>> >> >> using mmap() and>> writes or reads first byte of each page of mapped file
>>>> >> >> with some data.>
>>>Note that ZFS is designed so that it interacts somewhat badly with
>>>mmap() and other kernel services which rely on coherency between VM and
>>>IO such as sendfile(). At the very best, you will have two in-kernel
>>>copies of all data buffers used with such interfaces, but there have
>>>been sporadic reports that there are other bugs with it.
>>>
>>>If you have a test server, I'd recommend you do the same test on UFS for
>>>comparison.
>>
>> Was going to try this... Thanks for reply.
>> _______________________________________________
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>
> Why do you think that disabling prefetch is an overhead?
>
>
> --
> George Kontostanos
> Aicom telecoms ltd
> http://www.aisecure.net>
>
>
> Well... not me though. System administrator >_> . I suppose because we have
> a big IO traffic.

Not for a highly random I/O environment.

-- 
George Kontostanos
Aicom telecoms ltdhttp://www.aisecure.net
> 

That's is a reason we don't disable. Our reads and writes mostly
contiguous. And that problem arises on machines with low RAM (8Gb and
less). Also for obvious reasons swap is disabled.