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

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Message-Id: <67060CC0-9F64-40ED-9467-572996ECF21F@whamcloud.com>
Date:	Mon, 23 Apr 2012 01:24:39 -0500
From:	Andreas Dilger <adilger@...mcloud.com>
To:	Zheng Liu <gnehzuil.liu@...il.com>
Cc:	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 2012-04-22, at 21:25, Zheng Liu <gnehzuil.liu@...il.com> wrote:

> On Fri, Apr 20, 2012 at 05:21:59AM -0600, Andreas Dilger wrote:
>> 
>> 
>> 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.
> 
> Thanks for pointing out this feature.  I have evaluated this feature in my
> benchmark, and it can dramatically improve the performance. :-)
> 
> BTW, out of curiosity, why not set this feature on default?

As mentioned previously, one drawback of depending on the checksums for transaction commit is that if one block in and of the older transactions is bad, then this will cause the bad block's transaction to be aborted, along with all of the later transactions.

By skipping the replay of many transactions after reboot (some of which may have already written to the filesystem before the crash) this may leave the filesystem in a very inconsistent state. 

A better solution. (which has been discussed, but not implemented yet) is to write the checksum for each block in the transaction, and only skip restoring the block(s) with a good checksum in an otherwise complete transaction. 

This would need to change the journal disk format, but might be a good time to do this with Darrick's other checksum patches. 

Cheers, Andreas--
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