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

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Date:	Sat, 28 Apr 2012 16:58:12 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	"Ted Ts'o" <tytso@....edu>
Cc:	"Darrick J. Wong" <djwong@...ibm.com>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Sunil Mushran <sunil.mushran@...cle.com>,
	Martin K Petersen <martin.petersen@...cle.com>,
	Greg Freemyer <greg.freemyer@...il.com>,
	Amir Goldstein <amir73il@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Mingming Cao <cmm@...ibm.com>,
	Joel Becker <jlbec@...lplan.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Coly Li <colyli@...il.com>
Subject: Re: [PATCH 15/23] jbd2: Change disk layout for metadata checksumming

On 2012-04-28, at 8:19, Ted Ts'o <tytso@....edu> wrote:

> On Tue, Mar 06, 2012 at 12:49:41PM -0800, Darrick J. Wong wrote:
>> @@ -177,11 +189,17 @@ typedef struct journal_block_tag_s
>>    __be32        t_blocknr;    /* The on-disk block number */
>>    __be32        t_flags;    /* See below */
>>    __be32        t_blocknr_high; /* most-significant high 32bits. */
>> +    __be32        t_checksum;    /* crc32c(uuid+seq+block) */
>> } journal_block_tag_t;
>> 
>> #define JBD2_TAG_SIZE32 (offsetof(journal_block_tag_t, t_blocknr_high))
>> #define JBD2_TAG_SIZE64 (sizeof(journal_block_tag_t))
> 
> There's a problem with this patch here --- we are changing the size of
> journal_block_tag_t, which is an on-disk data structure.  So for
> 64-bit journals, this represents a format change.  This means that if
> you have a 64-bit file system that needs to have its journal
> recovered, if the journal was written with an older kernel, and then
> we try to recover it with a new kernel, things won't be good.
> Similarly, for e2fsck's recovery code, it's not going to be able to
> recover 64-bit file systems using current coding, since this patch
> series changes the size of JBD2_TAG_SIZE64.
> 
> What we need to do is something like this:
> 
> #define JBD2_TAG_SIZE64 (offsetof(journal_block_tag_t, t_checksum))
> #define JBD2_TAG_SIZE_CSUM (sizeof(journal_block_tag_t))
> 
> And then change the code appropriately in e2fsprogs and in the kernel
> to use the correct tag size depending on the journal options.

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. 

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. 

Cheers, Andreas--
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