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SubjectRe: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority
On Thu, 2 Oct 2008 21:50:26 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> >
> > I also notice it's part of "file_update_time". Do we really need to
> > go all the way down to this level of synchronicity for that?
>
> Well, we've tossed that around many times but never implemented it.
> Once you get into the details it gets a bit nasty. Need to keep the
> dirtiness state in the VFS (or fs) inode, and going backwards from a
> plain old buffer_head at commit time isn't possible. We usually
> tempfixed the problem by adding increasingly fancy ways of not doing
> the atime update at all.

given that this is the write path, I was assuming this was mtime rather
than atime; doesn't change your answer though.
>
> Of course, fixing this running-vs-committing contention point would
> fix a lot more things than just atime updates.

yes clearly. It's waaay above my paygrade to hack on though; JBD is one
of those places in the kernel that scare me for doing fundamental
changes ;-(

>
> > (I also randomly wonder if we, in the write path, dirty the inode
> > twice, once for size once for item, and if we then also reserve two
> > slots in the journal for that.....
>
> That shouldn't be the case - once we have write access to the buffer
> it remains freely modifiable for the rest of the transaction period.
> I think.

I hope you're right otherwise we'd always hit this; once for the size
change, then block for the mtime. That would thoroughly suck; so much
so that you just must be right.



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