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

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Message-ID: <20110603004300.GE16306@thunk.org>
Date:	Thu, 2 Jun 2011 20:43:00 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Vivek Goyal <vgoyal@...hat.com>
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 08:27:14PM -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.

     	       	   	       	       	  - Ted
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