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

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Message-ID: <CAFgt=MC0NJC9fgGaS7Oe7LTpjDgcN-53Wx7TV-1Q=X3zFv73Ew@mail.gmail.com>
Date:	Thu, 18 Aug 2011 11:54:17 -0700
From:	Jiaying Zhang <jiayingz@...gle.com>
To:	Michael Tokarev <mjt@....msk.ru>
Cc:	"Ted Ts'o" <tytso@....edu>, Tao Ma <tm@....ma>,
	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)

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.

Jiaying

>
> Thank you!
>
> /mjt
>
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