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

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Date:	Fri, 19 Aug 2011 11:18:43 +0800
From:	Tao Ma <tm@....ma>
To:	Michael Tokarev <mjt@....msk.ru>
CC:	Ted Ts'o <tytso@....edu>, Jiaying Zhang <jiayingz@...gle.com>,
	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 Michael,
On 08/18/2011 02:49 PM, Michael Tokarev 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?
So could you please share with us how you test and your test result
with/without dioread_nolock? A quick test with fio and intel ssd does't
see much improvement here.

We are based on RHEL6, and dioread_nolock isn't there by now and a large
number of our product system use direct read and buffer write. So if
your test proves to be promising, I guess our company can arrange some
resources to try to work it out.

Thanks
Tao
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