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Message-ID: <20130111022907.GA7234@gmail.com> Date: Fri, 11 Jan 2013 10:29:07 +0800 From: Zheng Liu <gnehzuil.liu@...il.com> To: Jan Kara <jack@...e.cz> Cc: linux-ext4@...r.kernel.org Subject: Re: [RFC] call end_page_writeback after converting unwritten extents in ext4_end_io On Thu, Jan 10, 2013 at 03:47:19PM +0100, Jan Kara wrote: [cut...] > > Now I have an idea to solve this problem. We start a journal before submitting > > an io request rather than start it in ext4_convert_unwritten_extents(). The > > reason of starting a journal in ext4_convert_unwritten_extents() is that we need > > to calculate credits for journal. But as far as I understand the credits is not > > increased in this function because we have splitted extents before submitting > > this io request. A 'handle_t *handle' will be added into ext4_io_end_t, and it > > will be used in ext4_convert_unwritten_extents(). Then we can avoid to > > trigger a journal commit when starting a journal. > I'm actually already working on a solution. The disadvantage of starting > a transaction before IO submission is that the handle will hold transaction > open all the time until IO is finished and extent converted. So it can > effectively block any filesystem activity for a relatively long time. I've > already written a patch for JBD2 to allow transaction reservation - it > reserves blocks in the journal but they are not attached to a particular > transaction. Later during extent conversion we transform this reservation > into a real handle (without waiting for the journal so locking is OK). Great! > > The part I'm missing so far is adding transaction reservation into IO > submission path. That is actually somewhat tricky because we have to do it > before taking page locks and propagate the reserved handle all the way down > to the point where we allocate io_end. And furthermore we have to somehow > deal with the fact that IO to one extent can be split among multiple BIOs > (as it happens e.g. when an extent is longer than 512 KB which is usual > limit on BIO size) and thus multiple io_end structures are created and > extent is converted in parts (actually we didn't think about this problem > previously in extent conversion code). We don't know in advance how much > BIOs we'll need to write the extents (bio_add_page() decides when the BIO > is full and there are other constraints on BIO than just the total size) so > what we need to do it so have one io_end structure shared by all the BIOs > covering the extent. That will also save us from unnecessary splitting and > joining of extents for conversion. But doing that requires some changes to > io submission path which is why it's taking me longer than I'd like (plus I > have other obligations than just improving ext4 ;) But I'm working on it so > please stay tuned... Now all I need to do is to have a cup of coffee and wait your patches. :) Regards, - Zheng -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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