linux-ext4 - Re: Remarks regarding sparse

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Message-ID: <57337EF8.5090008@partition-saving.com>
Date:	Wed, 11 May 2016 20:50:32 +0200
From:	Damien Guibouret <damien.guibouret@...tition-saving.com>
To:	linux-ext4@...r.kernel.org
Subject: Re: Remarks regarding sparse_super2 feature

Theodore Ts'o wrote:
> On Sat, Apr 30, 2016 at 05:23:58PM +0200, Damien Guibouret wrote:
> 
[...]
> 
> 
>>For the other case (shrinking the fs) at line 856:
>>			if (last_bg > 1 &&
>>			    old_fs->super->s_backup_bgs[1] == old_last_bg)
>>				fs->super->s_backup_bgs[1] = last_bg;
>>what ensures the location where the new super block backup will be set is a
>>free block?
> 
> 
> This is handled by reserve_sparse2_last_group().  We have to deal this
> sort of thing whenever we need to do things like grow the group
> descriptors, and we will relocate data blocks as necessary to make
> room for blocks that have to be at specific locations.  (This means we
> have allocate new blocks for the blocks we are moving, copy the data
> blocks, and then update the inode(s) to point the new block
> locations.)
> 
> Cheers,
> 
> 						- Ted

Hello,

Thanks for the feedback.
For the last point, you're right, I did not look deep enough in the remaining 
part of the code (and it looks obvious that when shrinking the fs, there is some 
need to reallocate blocks).

Regards,

Damien
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