linux-ext4 - Re: Regular ext4 error warning with HD in USB dock

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Date:	Sat, 8 Jan 2011 00:12:27 +0200
From:	Amir Goldstein <amir73il@...il.com>
To:	"Ted Ts'o" <tytso@....edu>
Cc:	Ric Wheeler <rwheeler@...hat.com>,
	Rogier Wolff <R.E.Wolff@...wizard.nl>,
	Con Kolivas <kernel@...ivas.org>, adilger.kernel@...ger.ca,
	linux-ext4@...r.kernel.org
Subject: Re: Regular ext4 error warning with HD in USB dock

On Fri, Jan 7, 2011 at 11:07 PM, Amir Goldstein <amir73il@...il.com> wrote:
> On Fri, Jan 7, 2011 at 9:41 PM, Amir Goldstein <amir73il@...il.com> wrote:
>> On Fri, Jan 7, 2011 at 7:26 AM, Ted Ts'o <tytso@....edu> wrote:
>>> Am I missing something?  The kernel stores up to 3k worth of data, on
>>> a 4k block file system.  Whereas e2fsck patch blindly assume 2k worhth
>>> of data regardless of the block size.  The kernel patch looks ok, but
>>> the e2fsprogs patch seems badly broken....
>
> So it's not badly broken, it copies blocksize-2K, which is clumsily
> written like this:
> +       int len = ctx->fs->blocksize - 2*SUPERBLOCK_OFFSET;
>
> After that, only 4K block and 8K block will have a leftover,
> which will be copied from journal sb+1K to ext4 sb+1K.
> Yes, 2K blocks - no message buffer for you!
>
> The reason I am only copying 2K and throwing the extra K is this:
> print_message_buffer() is called from check_super_block(),
> *after* journal recovery, which was executed either by e2fsck itself
> or (and more likely in a errors=remount-ro situation) by ext4
> on read-only mount.
> In the latter case, the extra K must be discarded, so I saw no reason
> to write special code for the first case.
> Neither did I find a good reason to complicate the recording code
> and limit it to record only blocksize-2K.
>
>

Ted,

I have a suggestion how to use the wasted extra K.

As I pointed out in the past, the first/last_error_xxx statistics are most
likely to be lost in errors=panic and errors=remount-ro (journal
recovery will override super block)
If you record this information in the last K of journal sb (even copy
the entire ext4 sb),
you can then override ext4 sb with the most up-to-date error stats
after journal recovery.

Amir.
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