linux-ext4 - Re: [PATCH 2/4] e2fsprogs: Add undoe2fs

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Date:	Wed, 01 Aug 2007 13:22:30 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Andreas Dilger <adilger@...sterfs.com>
CC:	tytso@....edu, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 2/4] e2fsprogs: Add undoe2fs



Andreas Dilger wrote:
> On Aug 01, 2007  11:46 +0530, Aneesh Kumar K.V wrote:
>> Andreas Dilger wrote:
>>> On Aug 01, 2007  07:34 +0530, Aneesh Kumar K.V wrote:
>>>> undoe2fs can be used to replay the transaction saved
>>>> in the transaction file using undo I/O Manager
>>> This should save the mtime of the superblock, and only do the undo
>>> step if the filesystem hasn't changed.  Otherwise it could seriously
>>> corrupt the filesystem.
>> I am not sure i understand this. The Undo I/O manager tracks all the write
>> happening to the file system and copy the original content of the blocks to
>> the tdb file. Undoe2fs simply copies these blocks back to the file system. 
>>
>> That way if you look at undoe2fs it doesn't have any knowledge of the file
>> system at all.
>>
>> Can you let me know a use case where this will fail.
> 
> - modify filesystem with undo manager (e.g. inode resize)
> - mount filesystem, make changes, unmount
> - run undoe2fs to overwrite filesystem, corrupting it
> 


But that won't corrupt it. It will bring the file system back to
the state before inode resize. I understand that we may want to have

a) Don't replay if file system is mounted
b) Don't replay if UUID doesn't match


But i guess we should allow a replay if file system got changed afterwards.
Ofcourse the changes will no longer be available after the replay.

-aneesh
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