linux-ext4 - Re: Query about DIO/AIO WRITE throttling and ext4 serialization

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Message-ID: <20110603005403.GB27129@redhat.com>
Date:	Thu, 2 Jun 2011 20:54:03 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	"Ted Ts'o" <tytso@....edu>
Cc:	Dave Chinner <david@...morbit.com>, linux-ext4@...r.kernel.org
Subject: Re: Query about DIO/AIO WRITE throttling and ext4 serialization

On Thu, Jun 02, 2011 at 08:43:00PM -0400, Ted Ts'o wrote:
> On Thu, Jun 02, 2011 at 08:27:14PM -0400, Vivek Goyal wrote:
> > 
> > In this case only a single thread is doing IO continuously. I am assuming
> > if there is a database using XFS, it is not unreasonable to have prolonged
> > periods of continuous IO activity. In that case I think by above design
> > sync will not finish until and unless there is a momentary pause in IO. This
> > does not sound like the best design choice.
> 
> Sure, but under what circumstances would a database be blasting data
> using AIO/DIO in one thread, and calling fsync() in another thread?
> In practice I don't think this situation should ever arise.  If it
> did, the question of which writes could be considered safely on stable
> store and which would not would be undefined.  In fact, for most
> enterpise databases, they are using preallocated files, so there's no
> need at all to use fsync() and AIO/DIO at the same time.

In this case I had done "sync" while aio-stress was doing O_DIRECT writes.
I really don't have any real world example, I just cooked up a hypothetical
scenario.

Just wondering why ext4 and XFS behavior are different and which is a
more appropriate behavior. ext4 does not seem to be waiting for all
pending AIO/DIO to finish while XFS does.

Thanks
Vivek
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