linux-ext4 - Re: [PATCH] libext2fs: reserve exclude bitmap fields in group descriptor

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Date:	Fri, 16 Sep 2011 09:55:03 +0300
From:	Amir Goldstein <amir73il@...il.com>
To:	Andreas Dilger <adilger.kernel@...ger.ca>
Cc:	"Ted Ts'o" <tytso@....edu>, "Darrick J. Wong" <djwong@...ibm.com>,
	Sunil Mushran <sunil.mushran@...cle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Mingming Cao <cmm@...ibm.com>,
	Joel Becker <jlbec@...lplan.org>, linux-ext4@...r.kernel.org,
	Coly Li <colyli@...il.com>,
	Yongqiang Yang <xiaoqiangnk@...il.com>
Subject: Re: [PATCH] libext2fs: reserve exclude bitmap fields in group descriptor

On Thu, Sep 15, 2011 at 10:08 PM, Andreas Dilger
<adilger.kernel@...ger.ca> wrote:
> On 2011-09-15, at 7:47 AM, Amir Goldstein wrote:
>> On Thu, Sep 15, 2011 at 4:16 PM, Ted Ts'o <tytso@....edu> wrote:
>>> On Thu, Sep 15, 2011 at 09:50:20AM +0300, Amir Goldstein wrote:
>>>> -#define EXT2_FEATURE_COMPAT_EXCLUDE_INODE      0x0080
>>>> +/* #define EXT2_FEATURE_COMPAT_EXCLUDE_INODE   0x0080 not used */
>>>> +#define EXT2_FEATURE_COMPAT_EXCLUDE_BITMAP     0x0100
>>>
>>> Why this change?  Is it because you're already using 0x0100 in
>>> shipping systems?
>>
>> I am using 0x80 in shipping systems and it signifies something a bit
>> different then the proposed 0x100.
>>
>> EXCLUDE_INODE means that special inode 9 is used to reference exclude
>> bitmap blocks.  EXCLUDE_BITMAP means that exclude bitmap blocks are
>> referenced from group descriptors.
>> With this distinction it will be easier for me to make the migration.
>
> In that light, why not continue to use an inode to map the exclude bitmap
> blocks, where the bitmap offset is (group * blocksize), instead of
> explicitly listing all of the blocks in the group descriptor?  This is
> how the buddy bitmap works in memory only, but it could be done for the
> exclude bitmap on disk.
>
> The advantage of this is that it would allow the 32-bit bitmap checksums
> to both fit into the group descriptor.  The disadvantage is that there
> is a chance this inode would become corrupted and the location of the
> exclude bitmaps is lost.  I don't know how serious that is (e.g. if e2fsck
> could fix it by regenerating the bitmaps, or just deleting the snapshot).
>
> Cheers, Andreas
>

fsck can fix it. it just marks all blocks used by snapshot inodes.

Amir.
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