linux-ext4 - Re: [RFC] jbd2: reduce the number of writes when commiting a transacation

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Date:	Tue, 24 Apr 2012 21:27:07 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Jan Kara <jack@...e.cz>
Cc:	Andreas Dilger <adilger@...mcloud.com>,
	Zheng Liu <gnehzuil.liu@...il.com>,
	Andreas Dilger <adilger@...ger.ca>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC] jbd2: reduce the number of writes when commiting a
 transacation

On Tue, Apr 24, 2012 at 11:57:09PM +0200, Jan Kara wrote:
>   Also currently the async commit code has essentially unfixable bugs in
> handling of cache flushes as I wrote in
> http://www.spinics.net/lists/linux-ext4/msg30452.html. Because data blocks
> are not part of journal checksum, it can happen with async commit code that
> data is not safely on disk although transaction is completely committed. So
> async commit code isn't really safe to use unless you are fine with
> exposure of uninitialized data...

With the old journal checksum, the data blocks *are* part of the
journal checksum.  That's not the reason I haven't enabled it as a
default (even though it would close to double fs_mark benchmarks).
The main issue is that e2fsck doesn't deal intelligently if some
commit *other* than the last one has a bad intelligent.

With the new journal checksum patches, each individual data block has
its own checksum, so we don't need to discard the entire commit;
instead we can just drop the individual block(s) that have a bad
checksum, and then force a full fsck run afterwards.

						- Ted
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