linux-kernel - Re: [PATCH] Give kjournald a IOPRIO_CLASS

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Date:	Mon, 6 Oct 2008 14:18:29 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Aaron Carroll <aaronc@...ato.unsw.edu.au>
Cc:	Bodo Eggert <7eggert@....de>, Jens Axboe <jens.axboe@...cle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority

On Sat, Oct 04, 2008 at 05:45:00PM +1000, Aaron Carroll wrote:
> Dave Chinner wrote:
>> On Thu, Oct 02, 2008 at 05:32:04PM +0200, Bodo Eggert wrote:
>>> Sounds like you need a priority class besides sync and async.
>>
>> There's BIO_META now as well, which I was testing at the same time
>> as RT priority. Marking all the metadata I/O as BIO_META did help,
>> but once again I never got to determining if that was a result of
>> the different tagging or the priority increase.
>
> What exactly do you want META to mean?  Strict prioritisation over
> all other non-META requests, or just more frequent and/or larger
> dispatches?  Should META requests be sorted?

The real question is "what was it supposed to mean"? AFAICT, it was
added to a couple of filesystems to be used to tag superblock read
I/O. Why - I don't know - there's a distinct lack of documentation
surrounding these bio flags. :/

Realistically, I'm not sure that having a separate queue for
BIO_META will buy us anything, given that noop is quite often the
fastest scheduler for XFS because it enables interleaved metadata
I/O to be merged with data I/O. Like I said, I was not able to spend
the time to determine exactly how BIO_META affected I/O patterns, so
I can't really comment on whether it is really necessary or not.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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