linux-ext4 - Re: [PATCH 0/5 v2] add extent status tree caching

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Message-ID: <51E88ECD.3040806@redhat.com>
Date:	Thu, 18 Jul 2013 19:56:45 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	"Theodore Ts'o" <tytso@....edu>
CC:	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	Zheng Liu <gnehzuil.liu@...il.com>
Subject: Re: [PATCH 0/5 v2] add extent status tree caching

On 7/18/13 1:53 PM, Theodore Ts'o wrote:
> On Thu, Jul 18, 2013 at 01:35:24PM -0500, Eric Sandeen wrote:
>>> (Should we do this all the time, instead of when the application
>>> explicitly requests it?  Maybe; there could be cases with very large,
>>> fragmented files accessed by an application such as "file" is only needs
>>> to look at a small subset of the file where this could result in an
>>> unnecessary work and memory allocated.  OTOH, 95%+ of the time this
>>> would probably be a win...)
>>
>> I'd say yes, we should - maybe not in all cases but if you need it for
>> AIO, try to make it "all the time" at least for that AIO?
> 
> The problem is we don't know that we're doing AIO until we see the
> first io_submit(2) call.  With this patch series, we'll pull the
> contents of the entire leaf tree block into extent cache, but if the
> extent tree is larger than that, if we read in the entire extent tree
> on the first AIO request, then that first request will delayed even
> more, and it's not clear that's a good thing.

Is blocking on a pre-AIO ioctl better than blocking on the
first AIO?

-Eric
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