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Date:	Fri, 20 Apr 2012 05:21:59 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Zheng Liu <gnehzuil.liu@...il.com>
Cc:	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC] jbd2: reduce the number of writes when commiting a transacation

On 2012-04-20, at 5:06 AM, Zheng Liu wrote:
> In this thread[1], I found a defect in jbd2 because it needs two wrties
> to finish a transacation because it writes journal header and data to
> disk and it will write commit to disk after above writes are done.
> AFAIK, in jbd2, it will call submit_bh twice at least to write the data
> because journal header, data and commit are stored in different
> buffer_heads.  If we don't call them separately, these calls might be
> out of order.  Obviously, it must ensure that journal header and data are written before commit.  But this brings a huge overhead in this
> benchmark[2].  So, IMHO, if we could use *bio* to store these data
> rather than buffer_head, we could avoid this overhead because we can
> call submit_bio only once to write all of data, which contains journal
> header, data and commit.  Here is an issue that I don't determine.  If
> we use submit_bio to write journal data, it will make all of data with
> WRITE_FLUSH_FUA flag.  But now there is only commit data with this flag.

The reason that there are two separate writes is because if the write
of the commit block is reordered before the journal data, and only the
commit block is written before a crash (data is lost), then the journal
replay code may incorrectly think that the transaction is complete and
copy the unwritten (garbage) block to the wrong place.

I think there is potentially an existing solution to this problem,
which is the async journal commit feature.  It adds checksums to the
journal commit block, which allows verifying that all blocks were
written to disk properly even if the commit block is submitted at
the same time as the journal data blocks.

One problem with this implementation is that if an intermediate
journal commit has a data corruption (i.e. checksum of all data
blocks does not match the commit block), then it is not possible
to know which block(s) contain bad data.  After that, potentially
many thousands of other operations may be lost.

We discussed a scheme to store a separate checksum for each block
in a transaction, by storing a 16-bit checksum (likely the low
16 bits of CRC32c) into the high flags word for each block.  Then,
if one or more blocks is corrupted, it is possible to skip replay
of just those blocks, and potentially they will even be overwritten
by blocks in a later transaction, requiring no e2fsck at all.

> I am not sure whether or not it brings some other unpridictable
> problems. :(
> 
> Please feel free to comment this RFC.  Thank you.
> 
> 1. http://www.spinics.net/lists/linux-ext4/msg31637.html
> 2. benchmark: time for((i=0;i<2000;i++)); do \
> 		dd if=/dev/zero of=/mnt/sda1/testfile conv=notrunc bs=4k \
> 		count=1 seek=`expr $i \* 16` oflag=sync,direct 2>/dev/null; \
> 		done
> 
> Regards,
> Zheng
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Cheers, Andreas





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