linux-kernel - Re: Linux disk performance.

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Date:	Mon, 18 Dec 2006 23:54:21 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Manish Regmi <regmi.manish@...il.com>
CC:	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, kernelnewbies@...linux.org
Subject: Re: Linux disk performance.

Manish Regmi wrote:
> On 12/18/06, Arjan van de Ven <arjan@...radead.org> wrote:
> 
>> if you want truely really smooth writes you'll have to work for it,
>> since "bumpy" writes tend to be better for performance so naturally the
>> kernel will favor those.
>>
>> to get smooth writes you'll need to do a threaded setup where you do an
>> msync/fdatasync/sync_file_range on a frequent-but-regular interval from
>> a thread. Be aware that this is quite likely to give you lower maximum
>> performance than the batching behavior though.
>>
> 
> Thanks...
> 
> But isn't O_DIRECT supposed to bypass buffering in Kernel?
> Doesn't it directly write to disk?
> I tried to put fdatasync() at regular intervals but there was no
> visible effect.
> 

I don't know exactly how to interpret the numbers you gave, but
they look like they might be a (HZ quantised) delay coming from
block layer plugging.

O_DIRECT bypasses caching, but not (all) buffering.

Not sure whether the block layer can handle an unplug_delay set
to 0, but that might be something to try (see block/ll_rw_blk.c).

-- 
SUSE Labs, Novell Inc.
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