linux-ext4 - Re: [PATCH 15/23] jbd2: Change disk layout for metadata checksumming

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Date:	Sun, 29 Apr 2012 22:08:22 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Ted Ts'o <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org, Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <jlbec@...lplan.org>
Subject: Re: [PATCH 15/23] jbd2: Change disk layout for metadata checksumming

On 2012-04-29, at 1:39 PM, Ted Ts'o wrote:
> On Sat, Apr 28, 2012 at 04:58:12PM -0600, Andreas Dilger wrote:
>> 
>> I thought we originally discussed using the high 16 bits of the
>> t_flags field to store the checksum?  This would avoid the need to
>> change the disk format.
> 
> I don't recall that suggestion, but I like it.  One thing that will
> get subtle about this is that t_flags is stored big-endian (jbd/jbd2
> data structures are stored be, but the data structures in ext4 proper
> are stored le; sigh).   So we'd have to do something like this:
> 
> typedef struct journal_block_tag_s
> {
> 	__u32		t_blocknr;	/* The on-disk block number */
> 	__u16		t_checksum;	/* 16-bit checksum */
> 	__u16		t_flags;	/* See below */
> 	__u32		t_blocknr_high; /* most-significant high 32bits. */
> } journal_block_tag_t;
> 
> ... and then make sure we change all of the places that access t_flags
> using cpu_to_be32() and be32_to_cpu() get changed to the 16-bit
> variant.

Sigh...

>> Since there is still a whole transaction checksum, it isn't so
>> critical that the per-block checksum be strong.
>> 
>> One idea is to do the crc32c for each block, then store the high 16
>> bits into t_flags, and checksum the full 32-bit per-block checksums
>> to make the commit block checksum, to avoid having to do the block
>> checksums twice.
> 
> It's not critical because the hard drive is doing its own ECC.  So I'm
> not that worried about detecting a large burst of bit errors, which is
> the main advantage of using a larger CRC.  I'm more worried about a
> disk block getting written to the wrong place, or not getting written
> at all.  So whether the chance of detecting a wrong block is 99.9985%
> at all (w/ a 16-bit checksum) or 99.9999% (with a 32-bit checksum),
> at all, either is fine.
> 
> I'm not even sure I would worry combining the per-block checksums into
> the commit block checksum.  In the rare case where there is an error
> not detected by the 16-bit checksum which is detected in the commit
> checksum, what are we supposed to do?  Throw away the entire commit
> again?  Just simply testing to see what we do in this rare case is
> going to be interesting / painful.

The main reason to create the commit block checksum out of the
per-block checksums is to avoid having to checksum the data blocks
themselves twice.  That would be a significant overhead to compute,
say, 4096 * 4kB = 16MB of block data, while computing the commit
block checksum out of 4096 * 2 bytes = 8kB of the 16-bit checksums
is relatively minor.

Cheers, Andreas





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