linux-ext4 - Re: DIO process stuck apparently due to dioread

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Message-ID: <4E4DD679.8020603@tao.ma>
Date:	Fri, 19 Aug 2011 11:20:25 +0800
From:	Tao Ma <tm@....ma>
To:	Jiaying Zhang <jiayingz@...gle.com>
CC:	Michael Tokarev <mjt@....msk.ru>, Ted Ts'o <tytso@....edu>,
	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
	sandeen@...hat.com
Subject: Re: DIO process stuck apparently due to dioread_nolock (3.0)

Hi Ted and Jiaying,
On 08/19/2011 02:54 AM, Jiaying Zhang wrote:
> On Wed, Aug 17, 2011 at 11:49 PM, Michael Tokarev <mjt@....msk.ru> wrote:
>> 17.08.2011 21:02, Ted Ts'o wrote:
>> []
>>> What I'd like to do long-term here is to change things so that (a)
>>> instead of instantiating the extent as uninitialized, writing the
>>> data, and then doing the uninit->init conversion to writing the data
>>> and then instantiated the extent as initialzied.  This would also
>>> allow us to get rid of data=ordered mode.  And we should make it work
>>> for fs block size != page size.
>>>
>>> It means that we need a way of adding this sort of information into an
>>> in-memory extent cache but which isn't saved to disk until the data is
>>> written.  We've also talked about adding the information about whether
>>> an extent is subject to delalloc as well, so we don't have to grovel
>>> through the page cache and look at individual buffers attached to the
>>> pages.  And there are folks who have been experimenting with an
>>> in-memory extent tree cache to speed access to fast PCIe-attached
>>> flash.
>>>
>>> It seems to me that if we're careful a single solution should be able
>>> to solve all of these problems...
>>
>> What about current situation, how do you think - should it be ignored
>> for now, having in mind that dioread_nolock isn't used often (but it
>> gives _serious_ difference in read speed), or, short term, fix this
>> very case which have real-life impact already, while implementing a
>> long-term solution?
> I plan to send my patch as a bandaid fix. It doesn't solve the fundamental
> problem but I think it helps close the race you saw on your test. In the long
> term, I agree that we should think about implementing an extent tree cache
> and use it to hold pending uninitialized-to-initialized extent conversions.
Does Google has some plan of doing it recently? We used a large number
of direct read, and we can arrange some resources to try to work it out.

Thanks
Tao
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