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Date:	Thu, 21 May 2015 20:08:17 -0400
From:	Phillip Susi <psusi@...ntu.com>
To:	Theodore Ts'o <tytso@....edu>
CC:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: Unused block group, but all blocks not free?

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On 05/21/2015 07:59 PM, Theodore Ts'o wrote:
> Ah, so it's pretty self-explanatory.  From the dumpe2fs of the
> image:
> 
> Group 25: (Blocks 819200-851967) csum 0x45a8 [INODE_UNINIT,
> BLOCK_UNINIT, ITABLE_ZEROED] ^^^^^^^^^^^^^ Backup superblock at
> 819200, Group descriptors at 819201-819201 ^^^^^^
> ^^^^^^^^^^^^^ Reserved GDT blocks at 819202-819584 ^^^^^^^^^^^^^ 
> Block bitmap at 524297 (bg #16 + 9) Inode bitmap at 524313 (bg #16
> + 25) Inode table at 528928-529439 (bg #16 + 4640) 32383 free
> blocks, 8192 free inodes, 0 directories, 8192 unused inodes Free
> blocks: 819585-851967 Free inodes: 204801-212992

Interesting... I don't get that information from debugfs.
Specifically it doesn't give the blocks that comprise the group ( but
that is calculated easily enough ), nor list which blocks are reserved
for the GDT.  I'm using 1.42.12, is that a more recent feature?

I had actually just figured out that 25 is where the superblock and
GDT backups are, and therefore, should also contain part of the resize
inode.  It seems that e2defrag adds the blocks owned by reserved
inodes after checking the free count so I'll have to rejigger it to do
that first.

I wonder though, why I didn't see these problems the last time I used
e2defrag, which was probably 2 years ago and was on a filesystem with
a resize inode, and flex_bg and uninitialized bitmaps.  At the time it
seemed that uninitialized bitmaps were not used on groups with blocks
used for these sorts of things.  I guess that must also be a more
recent change.


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