Re: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority

[Dave Chinner <david@fromorbit.com>]

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@fromorbit.com