ZFS and mem management

George Kontostanos gkontos.mail at gmail.com
Wed Feb 15 16:06:35 UTC 2012


2012/2/15 Pavlo <devgs at ukr.net>:
>
>
> --- 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 ltd
> http://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.
>

I guess then that a memory upgrade should be scheduled.

-- 
George Kontostanos
Aicom telecoms ltd
http://www.aisecure.net


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