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I got "block bitmap differences" when doing e2fsck. I do e2fsck after i unmount that partition. My question is: what exactly is "bitmap difference". The difference is between what to what?

Thank you!

~ # e2fsck -c /dev/sda1
e2fsck 1.42.5 (29-Jul-2012)
ext2fs_check_if_mount: Can't check if filesystem is mounted due to missing mtab file while determining whether /dev/sda1 is mounted.
Checking for bad blocks (read-only test): done
/dev/sda1: Updating bad block inode.
Pass 1: Checking inodes, blocks, and sizes
Inode 81738, i_blocks is 16, should be 8.  Fix<y>? yes
Pass 2: Checking directory structure
Entry 'collector_prof.txt' in /etc/idirect (81624) has deleted/unused inode 81762.  Clear<y>? yes
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
Block bitmap differences:  -(346115--346116)
Fix<y>? yes
Free blocks count wrong for group #10 (32124, counted=32126).
Fix<y>? yes
Free blocks count wrong (435610, counted=435612).
Fix<y>? yes
Inode bitmap differences:  -81762
Fix<y>? yes
Free inodes count wrong for group #10 (7998, counted=7999).
Fix<y>? yes
Free inodes count wrong (121381, counted=121382).
Fix<y>? yes

/dev/sda1: ***** FILE SYSTEM WAS MODIFIED *****
/dev/sda1: 1018/122400 files (0.1% non-contiguous), 53599/489211 blocks
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Regularly, or just this one particular time? The "block bitmap differences" is not really an "inode error", methinks: virtualblueness.net/Ext2fs-overview/Ext2fs-overview-0.1-8.html There are redundant copies of stuff like that, so probably one of them was corrupted/wrong. – goldilocks Mar 20 '13 at 14:42
@goldilocks I use ext2 with unsychronized mode. Power off a lot. so it happens sometimes – Anders Lind Mar 20 '13 at 15:14
up vote 3 down vote accepted

Googling, I found none other than Theodore Tso talking about it:


The relevant part of that:

This means the filesystem noticed some discrepancy (for example, when
freeing a block, it noticed that the block bitmap already showed the
block as being not in use, which should never happen and indicates
filesystem corruption).

A "block bitmap" is just an on-disk data structure indicating which blocks in a particular group are allocated and which aren't. Between that and Theodore's explanation I would say that it probably found a block in use by an inode that wasn't marked as such in the bitmap for that block's group.

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