linux-ext4 - Re: flashing large eMMC partitions with ext4

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Message-ID: <CAOQ4uxiFaEvC0WpZH_bVp-Thhv0TMADNabd2pcr9pENfufDLoA@mail.gmail.com>
Date:	Tue, 2 Aug 2011 10:42:22 +0300
From:	Amir Goldstein <amir73il@...il.com>
To:	Round Robinjp <roundrobinjp@...oo.co.jp>
Cc:	Dilger Andreas <aedilger@...il.com>, "Ted Ts'o" <tytso@....edu>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: flashing large eMMC partitions with ext4

On Tue, Aug 2, 2011 at 9:53 AM, Round Robinjp <roundrobinjp@...oo.co.jp> wrote:
> Amir
>
>> create the initial a.img 4G and specify 1G to mkfs.
>> so the loop device size is 4G and you can online resize the fs to 4G.
>
> I think this time I have been able to achieve what I was
> trying to.
>

Yes, you have :-)

> After resize, there are 1048576 blocks, as expected.
> After truncating to 1G, dumpe2fs gives error, but I guess
> this is also expected behaviour.
>
> But after extending to 4G, e2fsck makes some complain.
> I guess this is not expected behaviour, is it?
>

it is expected when you use the current resize2fs,
which does not respect the flex_bg layout, so new group block bitmaps
are allocated beyond the 1G border and initialized.
if you use Yongqiang's new 64bit resize patches, the final fsck won't complain.
unfortunately for you, those patches have not been merged to the kernel yet,
so you will have to either build your own ext4 module or wait at least until
kernel 3.2 is released to have it in mainline.

It is actually quite simple to fix the 1G image, so it will pass fsck
after truncate -s 4G.
All it takes it setting the BLOCK_UNINIT flag in groups 8-31
this should be possible to do with debugfs (or write a small tool to do it).
if I have time, it will try it myself and post the instructions.


> However, now df shows 4G as the fs size, which is as expected.
>
> Log:
> --------------------------------
> --------------------------------
> --------------------------------
> $ dd if=/dev/zero of=a.img bs=4K count=1024K
> 1048576+0 records in
> 1048576+0 records out
> 4294967296 bytes (4.3 GB) copied, 14.2708 s, 301 MB/s
>
> $ mkfs.ext4 -O ^has_journal,flex_bg -G 32 -E resize=4G -b 4096 a.img 262144
> mke2fs 1.42-WIP (02-Jul-2011)
> a.img is not a block special device.
> Proceed anyway? (y,n) y
> Filesystem label=
> OS type: Linux
> Block size=4096 (log=2)
> Fragment size=4096 (log=2)
> Stride=0 blocks, Stripe width=0 blocks
> 65536 inodes, 262144 blocks
> 13107 blocks (5.00%) reserved for the super user
> First data block=0
> Maximum filesystem blocks=268435456
> 8 block groups
> 32768 blocks per group, 32768 fragments per group
> 8192 inodes per group
> Superblock backups stored on blocks:
>        32768, 98304, 163840, 229376
>
> Allocating group tables: done
> Writing inode tables: done
> Writing superblocks and filesystem accounting information: done
>
> This filesystem will be automatically checked every 0 mounts or
> 0 days, whichever comes first.  Use tune2fs -c or -i to override.
>
> $ sudo mount -t ext4 -o loop a.img /mnt/
>
> $ cp -a /usr/src/* /mnt/
>
> $ sudo resize2fs /dev/loop0
> resize2fs 1.42-WIP (02-Jul-2011)
> Filesystem at /dev/loop0 is mounted on /mnt; on-line resizing required
> old desc_blocks = 1, new_desc_blocks = 1
> Performing an on-line resize of /dev/loop0 to 1048576 (4k) blocks.
> The filesystem on /dev/loop0 is now 1048576 blocks long.
>
> $ sudo tune2fs -O has_journal /dev/loop0
> tune2fs 1.42-WIP (02-Jul-2011)
> Creating journal inode: done
> This filesystem will be automatically checked every 20 mounts or
> 0 days, whichever comes first.  Use tune2fs -c or -i to override.
>
> $ sudo umount /mnt
>
> $ dumpe2fs a.img
> dumpe2fs 1.42-WIP (02-Jul-2011)
> Filesystem volume name:   <none>
> Last mounted on:          /mnt
> Filesystem UUID:          f5a32b70-423d-448a-9550-90d67ff8b247
> Filesystem magic number:  0xEF53
> Filesystem revision #:    1 (dynamic)
> Filesystem features:      has_journal ext_attr resize_inode dir_index filetype extent flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize
> Filesystem flags:         signed_directory_hash
> Default mount options:    user_xattr acl
> Filesystem state:         clean
> Errors behavior:          Continue
> Filesystem OS type:       Linux
> Inode count:              262144
> Block count:              1048576
> Reserved block count:     52419
> Free blocks:              975516
> Free inodes:              241279
> First block:              0
> Block size:               4096
> Fragment size:            4096
> Reserved GDT blocks:      63
> Blocks per group:         32768
> Fragments per group:      32768
> Inodes per group:         8192
> Inode blocks per group:   512
> Flex block group size:    32
> Filesystem created:       Tue Aug  2 14:55:16 2011
> Last mount time:          Tue Aug  2 14:55:24 2011
> Last write time:          Tue Aug  2 15:33:06 2011
> Mount count:              1
> Maximum mount count:      20
> Last checked:             Tue Aug  2 14:55:16 2011
> Check interval:           0 (<none>)
> Lifetime writes:          406 kB
> Reserved blocks uid:      0 (user root)
> Reserved blocks gid:      0 (group root)
> First inode:              11
> Inode size:               256
> Required extra isize:     28
> Desired extra isize:      28
> Journal inode:            20500
> Default directory hash:   half_md4
> Directory Hash Seed:      6aa182f8-2ce9-4b90-9138-51b2fcf09ff0
> Journal features:         (none)
> Journal size:             128M
> Journal length:           32768
> Journal sequence:         0x00000001
> Journal start:            0
>
>
> Group 0: (Blocks 0-32767) [ITABLE_ZEROED]
>  Checksum 0x6170, unused inodes 0
>  Primary superblock at 0, Group descriptors at 1-1
>  Reserved GDT blocks at 2-64
>  Block bitmap at 65 (+65), Inode bitmap at 97 (+97)
>  Inode table at 129-640 (+129)
>  24746 free blocks, 0 free inodes, 3 directories
>  Free blocks: 8022-32767
>  Free inodes:
> Group 1: (Blocks 32768-65535) [ITABLE_ZEROED]
>  Checksum 0x4df3, unused inodes 0
>  Backup superblock at 32768, Group descriptors at 32769-32769
>  Reserved GDT blocks at 32770-32832
>  Block bitmap at 66 (bg #0 + 66), Inode bitmap at 98 (bg #0 + 98)
>  Inode table at 641-1152 (bg #0 + 641)
>  9128 free blocks, 0 free inodes, 975 directories
>  Free blocks: 49152-57343, 60376-60383, 60512-61439
>  Free inodes:
> Group 2: (Blocks 65536-98303) [ITABLE_ZEROED]
>  Checksum 0x7b0d, unused inodes 4076
>  Block bitmap at 67 (bg #0 + 67), Inode bitmap at 99 (bg #0 + 99)
>  Inode table at 1153-1664 (bg #0 + 1153)
>  4096 free blocks, 4076 free inodes, 2302 directories, 4076 unused inodes
>  Free blocks: 94208-98303
>  Free inodes: 20501-24576
> Group 3: (Blocks 98304-131071) [ITABLE_ZEROED]
>  Checksum 0x69b9, unused inodes 7865
>  Backup superblock at 98304, Group descriptors at 98305-98305
>  Reserved GDT blocks at 98306-98368
>  Block bitmap at 68 (bg #0 + 68), Inode bitmap at 100 (bg #0 + 100)
>  Inode table at 1665-2176 (bg #0 + 1665)
>  32703 free blocks, 7865 free inodes, 327 directories, 7865 unused inodes
>  Free blocks: 98369-131071
>  Free inodes: 24904-32768
> Group 4: (Blocks 131072-163839) [ITABLE_ZEROED]
>  Checksum 0x12e8, unused inodes 8154
>  Block bitmap at 69 (bg #0 + 69), Inode bitmap at 101 (bg #0 + 101)
>  Inode table at 2177-2688 (bg #0 + 2177)
>  32768 free blocks, 8154 free inodes, 38 directories, 8154 unused inodes
>  Free blocks: 131072-163839
>  Free inodes: 32807-40960
> Group 5: (Blocks 163840-196607) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0xe3c3, unused inodes 8192
>  Backup superblock at 163840, Group descriptors at 163841-163841
>  Reserved GDT blocks at 163842-163904
>  Block bitmap at 70 (bg #0 + 70), Inode bitmap at 102 (bg #0 + 102)
>  Inode table at 2689-3200 (bg #0 + 2689)
>  32703 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 163905-196607
>  Free inodes: 40961-49152
> Group 6: (Blocks 196608-229375) [INODE_UNINIT, BLOCK_UNINIT, ITABLE_ZEROED]
>  Checksum 0xaac1, unused inodes 8192
>  Block bitmap at 71 (bg #0 + 71), Inode bitmap at 103 (bg #0 + 103)
>  Inode table at 3201-3712 (bg #0 + 3201)
>  32768 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 196608-229375
>  Free inodes: 49153-57344
> Group 7: (Blocks 229376-262143) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0xa250, unused inodes 8192
>  Backup superblock at 229376, Group descriptors at 229377-229377
>  Reserved GDT blocks at 229378-229440
>  Block bitmap at 72 (bg #0 + 72), Inode bitmap at 104 (bg #0 + 104)
>  Inode table at 3713-4224 (bg #0 + 3713)
>  32703 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 229441-262143
>  Free inodes: 57345-65536
> Group 8: (Blocks 262144-294911) [ITABLE_ZEROED]
>  Checksum 0x99e5, unused inodes 0
>  Block bitmap at 262144 (+0), Inode bitmap at 262145 (+1)
>  Inode table at 262146-262657 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 262658-294911
>  Free inodes: 65537-73728
> Group 9: (Blocks 294912-327679) [ITABLE_ZEROED]
>  Checksum 0xf08a, unused inodes 0
>  Backup superblock at 294912, Group descriptors at 294913-294913
>  Reserved GDT blocks at 294914-294976
>  Block bitmap at 294977 (+65), Inode bitmap at 294978 (+66)
>  Inode table at 294979-295490 (+67)
>  32189 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 295491-327679
>  Free inodes: 73729-81920
> Group 10: (Blocks 327680-360447) [ITABLE_ZEROED]
>  Checksum 0xc787, unused inodes 0
>  Block bitmap at 327680 (+0), Inode bitmap at 327681 (+1)
>  Inode table at 327682-328193 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 328194-360447
>  Free inodes: 81921-90112
> Group 11: (Blocks 360448-393215) [ITABLE_ZEROED]
>  Checksum 0xe8b6, unused inodes 0
>  Block bitmap at 360448 (+0), Inode bitmap at 360449 (+1)
>  Inode table at 360450-360961 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 360962-393215
>  Free inodes: 90113-98304
> Group 12: (Blocks 393216-425983) [ITABLE_ZEROED]
>  Checksum 0x2521, unused inodes 0
>  Block bitmap at 393216 (+0), Inode bitmap at 393217 (+1)
>  Inode table at 393218-393729 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 393730-425983
>  Free inodes: 98305-106496
> Group 13: (Blocks 425984-458751) [ITABLE_ZEROED]
>  Checksum 0x0a10, unused inodes 0
>  Block bitmap at 425984 (+0), Inode bitmap at 425985 (+1)
>  Inode table at 425986-426497 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 426498-458751
>  Free inodes: 106497-114688
> Group 14: (Blocks 458752-491519) [ITABLE_ZEROED]
>  Checksum 0x7b43, unused inodes 0
>  Block bitmap at 458752 (+0), Inode bitmap at 458753 (+1)
>  Inode table at 458754-459265 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 459266-491519
>  Free inodes: 114689-122880
> Group 15: (Blocks 491520-524287) [ITABLE_ZEROED]
>  Checksum 0x5472, unused inodes 0
>  Block bitmap at 491520 (+0), Inode bitmap at 491521 (+1)
>  Inode table at 491522-492033 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 492034-524287
>  Free inodes: 122881-131072
> Group 16: (Blocks 524288-557055) [ITABLE_ZEROED]
>  Checksum 0xd378, unused inodes 0
>  Block bitmap at 524288 (+0), Inode bitmap at 524289 (+1)
>  Inode table at 524290-524801 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 524802-557055
>  Free inodes: 131073-139264
> Group 17: (Blocks 557056-589823) [ITABLE_ZEROED]
>  Checksum 0xfc49, unused inodes 0
>  Block bitmap at 557056 (+0), Inode bitmap at 557057 (+1)
>  Inode table at 557058-557569 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 557570-589823
>  Free inodes: 139265-147456
> Group 18: (Blocks 589824-622591) [ITABLE_ZEROED]
>  Checksum 0x8d1a, unused inodes 0
>  Block bitmap at 589824 (+0), Inode bitmap at 589825 (+1)
>  Inode table at 589826-590337 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 590338-622591
>  Free inodes: 147457-155648
> Group 19: (Blocks 622592-655359) [ITABLE_ZEROED]
>  Checksum 0xa22b, unused inodes 0
>  Block bitmap at 622592 (+0), Inode bitmap at 622593 (+1)
>  Inode table at 622594-623105 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 623106-655359
>  Free inodes: 155649-163840
> Group 20: (Blocks 655360-688127) [ITABLE_ZEROED]
>  Checksum 0x6fbc, unused inodes 0
>  Block bitmap at 655360 (+0), Inode bitmap at 655361 (+1)
>  Inode table at 655362-655873 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 655874-688127
>  Free inodes: 163841-172032
> Group 21: (Blocks 688128-720895) [ITABLE_ZEROED]
>  Checksum 0x408d, unused inodes 0
>  Block bitmap at 688128 (+0), Inode bitmap at 688129 (+1)
>  Inode table at 688130-688641 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 688642-720895
>  Free inodes: 172033-180224
> Group 22: (Blocks 720896-753663) [ITABLE_ZEROED]
>  Checksum 0x31de, unused inodes 0
>  Block bitmap at 720896 (+0), Inode bitmap at 720897 (+1)
>  Inode table at 720898-721409 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 721410-753663
>  Free inodes: 180225-188416
> Group 23: (Blocks 753664-786431) [ITABLE_ZEROED]
>  Checksum 0x1eef, unused inodes 0
>  Block bitmap at 753664 (+0), Inode bitmap at 753665 (+1)
>  Inode table at 753666-754177 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 754178-786431
>  Free inodes: 188417-196608
> Group 24: (Blocks 786432-819199) [ITABLE_ZEROED]
>  Checksum 0xeaf3, unused inodes 0
>  Block bitmap at 786432 (+0), Inode bitmap at 786433 (+1)
>  Inode table at 786434-786945 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 786946-819199
>  Free inodes: 196609-204800
> Group 25: (Blocks 819200-851967) [ITABLE_ZEROED]
>  Checksum 0x839c, unused inodes 0
>  Backup superblock at 819200, Group descriptors at 819201-819201
>  Reserved GDT blocks at 819202-819264
>  Block bitmap at 819265 (+65), Inode bitmap at 819266 (+66)
>  Inode table at 819267-819778 (+67)
>  32189 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 819779-851967
>  Free inodes: 204801-212992
> Group 26: (Blocks 851968-884735) [ITABLE_ZEROED]
>  Checksum 0xb491, unused inodes 0
>  Block bitmap at 851968 (+0), Inode bitmap at 851969 (+1)
>  Inode table at 851970-852481 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 852482-884735
>  Free inodes: 212993-221184
> Group 27: (Blocks 884736-917503) [ITABLE_ZEROED]
>  Checksum 0xddfe, unused inodes 0
>  Backup superblock at 884736, Group descriptors at 884737-884737
>  Reserved GDT blocks at 884738-884800
>  Block bitmap at 884801 (+65), Inode bitmap at 884802 (+66)
>  Inode table at 884803-885314 (+67)
>  32189 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 885315-917503
>  Free inodes: 221185-229376
> Group 28: (Blocks 917504-950271) [ITABLE_ZEROED]
>  Checksum 0x5637, unused inodes 0
>  Block bitmap at 917504 (+0), Inode bitmap at 917505 (+1)
>  Inode table at 917506-918017 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 918018-950271
>  Free inodes: 229377-237568
> Group 29: (Blocks 950272-983039) [ITABLE_ZEROED]
>  Checksum 0x7906, unused inodes 0
>  Block bitmap at 950272 (+0), Inode bitmap at 950273 (+1)
>  Inode table at 950274-950785 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 950786-983039
>  Free inodes: 237569-245760
> Group 30: (Blocks 983040-1015807) [ITABLE_ZEROED]
>  Checksum 0x0855, unused inodes 0
>  Block bitmap at 983040 (+0), Inode bitmap at 983041 (+1)
>  Inode table at 983042-983553 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 983554-1015807
>  Free inodes: 245761-253952
> Group 31: (Blocks 1015808-1048575) [ITABLE_ZEROED]
>  Checksum 0x2764, unused inodes 0
>  Block bitmap at 1015808 (+0), Inode bitmap at 1015809 (+1)
>  Inode table at 1015810-1016321 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>  Free blocks: 1016322-1048575
>  Free inodes: 253953-262144
>
> $ truncate -s 1G a.img
>
> $ dumpe2fs a.img
> dumpe2fs 1.42-WIP (02-Jul-2011)
> Filesystem volume name:   <none>
> Last mounted on:          /mnt
> Filesystem UUID:          f5a32b70-423d-448a-9550-90d67ff8b247
> Filesystem magic number:  0xEF53
> Filesystem revision #:    1 (dynamic)
> Filesystem features:      has_journal ext_attr resize_inode dir_index filetype extent flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize
> Filesystem flags:         signed_directory_hash
> Default mount options:    user_xattr acl
> Filesystem state:         clean
> Errors behavior:          Continue
> Filesystem OS type:       Linux
> Inode count:              262144
> Block count:              1048576
> Reserved block count:     52419
> Free blocks:              975516
> Free inodes:              241279
> First block:              0
> Block size:               4096
> Fragment size:            4096
> Reserved GDT blocks:      63
> Blocks per group:         32768
> Fragments per group:      32768
> Inodes per group:         8192
> Inode blocks per group:   512
> Flex block group size:    32
> Filesystem created:       Tue Aug  2 14:55:16 2011
> Last mount time:          Tue Aug  2 14:55:24 2011
> Last write time:          Tue Aug  2 15:33:06 2011
> Mount count:              1
> Maximum mount count:      20
> Last checked:             Tue Aug  2 14:55:16 2011
> Check interval:           0 (<none>)
> Lifetime writes:          406 kB
> Reserved blocks uid:      0 (user root)
> Reserved blocks gid:      0 (group root)
> First inode:              11
> Inode size:               256
> Required extra isize:     28
> Desired extra isize:      28
> Journal inode:            20500
> Default directory hash:   half_md4
> Directory Hash Seed:      6aa182f8-2ce9-4b90-9138-51b2fcf09ff0
> Journal features:         (none)
> Journal size:             128M
> Journal length:           32768
> Journal sequence:         0x00000001
> Journal start:            0
>
>
> Group 0: (Blocks 0-32767) [ITABLE_ZEROED]
>  Checksum 0x6170, unused inodes 0
>  Primary superblock at 0, Group descriptors at 1-1
>  Reserved GDT blocks at 2-64
>  Block bitmap at 65 (+65), Inode bitmap at 97 (+97)
>  Inode table at 129-640 (+129)
>  24746 free blocks, 0 free inodes, 3 directories
> Group 1: (Blocks 32768-65535) [ITABLE_ZEROED]
>  Checksum 0x4df3, unused inodes 0
>  Backup superblock at 32768, Group descriptors at 32769-32769
>  Reserved GDT blocks at 32770-32832
>  Block bitmap at 66 (bg #0 + 66), Inode bitmap at 98 (bg #0 + 98)
>  Inode table at 641-1152 (bg #0 + 641)
>  9128 free blocks, 0 free inodes, 975 directories
> Group 2: (Blocks 65536-98303) [ITABLE_ZEROED]
>  Checksum 0x7b0d, unused inodes 4076
>  Block bitmap at 67 (bg #0 + 67), Inode bitmap at 99 (bg #0 + 99)
>  Inode table at 1153-1664 (bg #0 + 1153)
>  4096 free blocks, 4076 free inodes, 2302 directories, 4076 unused inodes
> Group 3: (Blocks 98304-131071) [ITABLE_ZEROED]
>  Checksum 0x69b9, unused inodes 7865
>  Backup superblock at 98304, Group descriptors at 98305-98305
>  Reserved GDT blocks at 98306-98368
>  Block bitmap at 68 (bg #0 + 68), Inode bitmap at 100 (bg #0 + 100)
>  Inode table at 1665-2176 (bg #0 + 1665)
>  32703 free blocks, 7865 free inodes, 327 directories, 7865 unused inodes
> Group 4: (Blocks 131072-163839) [ITABLE_ZEROED]
>  Checksum 0x12e8, unused inodes 8154
>  Block bitmap at 69 (bg #0 + 69), Inode bitmap at 101 (bg #0 + 101)
>  Inode table at 2177-2688 (bg #0 + 2177)
>  32768 free blocks, 8154 free inodes, 38 directories, 8154 unused inodes
> Group 5: (Blocks 163840-196607) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0xe3c3, unused inodes 8192
>  Backup superblock at 163840, Group descriptors at 163841-163841
>  Reserved GDT blocks at 163842-163904
>  Block bitmap at 70 (bg #0 + 70), Inode bitmap at 102 (bg #0 + 102)
>  Inode table at 2689-3200 (bg #0 + 2689)
>  32703 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
> Group 6: (Blocks 196608-229375) [INODE_UNINIT, BLOCK_UNINIT, ITABLE_ZEROED]
>  Checksum 0xaac1, unused inodes 8192
>  Block bitmap at 71 (bg #0 + 71), Inode bitmap at 103 (bg #0 + 103)
>  Inode table at 3201-3712 (bg #0 + 3201)
>  32768 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
> Group 7: (Blocks 229376-262143) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0xa250, unused inodes 8192
>  Backup superblock at 229376, Group descriptors at 229377-229377
>  Reserved GDT blocks at 229378-229440
>  Block bitmap at 72 (bg #0 + 72), Inode bitmap at 104 (bg #0 + 104)
>  Inode table at 3713-4224 (bg #0 + 3713)
>  32703 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
> Group 8: (Blocks 262144-294911) [ITABLE_ZEROED]
>  Checksum 0x99e5, unused inodes 0
>  Block bitmap at 262144 (+0), Inode bitmap at 262145 (+1)
>  Inode table at 262146-262657 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 9: (Blocks 294912-327679) [ITABLE_ZEROED]
>  Checksum 0xf08a, unused inodes 0
>  Backup superblock at 294912, Group descriptors at 294913-294913
>  Reserved GDT blocks at 294914-294976
>  Block bitmap at 294977 (+65), Inode bitmap at 294978 (+66)
>  Inode table at 294979-295490 (+67)
>  32189 free blocks, 8192 free inodes, 0 directories
> Group 10: (Blocks 327680-360447) [ITABLE_ZEROED]
>  Checksum 0xc787, unused inodes 0
>  Block bitmap at 327680 (+0), Inode bitmap at 327681 (+1)
>  Inode table at 327682-328193 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 11: (Blocks 360448-393215) [ITABLE_ZEROED]
>  Checksum 0xe8b6, unused inodes 0
>  Block bitmap at 360448 (+0), Inode bitmap at 360449 (+1)
>  Inode table at 360450-360961 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 12: (Blocks 393216-425983) [ITABLE_ZEROED]
>  Checksum 0x2521, unused inodes 0
>  Block bitmap at 393216 (+0), Inode bitmap at 393217 (+1)
>  Inode table at 393218-393729 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 13: (Blocks 425984-458751) [ITABLE_ZEROED]
>  Checksum 0x0a10, unused inodes 0
>  Block bitmap at 425984 (+0), Inode bitmap at 425985 (+1)
>  Inode table at 425986-426497 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 14: (Blocks 458752-491519) [ITABLE_ZEROED]
>  Checksum 0x7b43, unused inodes 0
>  Block bitmap at 458752 (+0), Inode bitmap at 458753 (+1)
>  Inode table at 458754-459265 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 15: (Blocks 491520-524287) [ITABLE_ZEROED]
>  Checksum 0x5472, unused inodes 0
>  Block bitmap at 491520 (+0), Inode bitmap at 491521 (+1)
>  Inode table at 491522-492033 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 16: (Blocks 524288-557055) [ITABLE_ZEROED]
>  Checksum 0xd378, unused inodes 0
>  Block bitmap at 524288 (+0), Inode bitmap at 524289 (+1)
>  Inode table at 524290-524801 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 17: (Blocks 557056-589823) [ITABLE_ZEROED]
>  Checksum 0xfc49, unused inodes 0
>  Block bitmap at 557056 (+0), Inode bitmap at 557057 (+1)
>  Inode table at 557058-557569 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 18: (Blocks 589824-622591) [ITABLE_ZEROED]
>  Checksum 0x8d1a, unused inodes 0
>  Block bitmap at 589824 (+0), Inode bitmap at 589825 (+1)
>  Inode table at 589826-590337 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 19: (Blocks 622592-655359) [ITABLE_ZEROED]
>  Checksum 0xa22b, unused inodes 0
>  Block bitmap at 622592 (+0), Inode bitmap at 622593 (+1)
>  Inode table at 622594-623105 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 20: (Blocks 655360-688127) [ITABLE_ZEROED]
>  Checksum 0x6fbc, unused inodes 0
>  Block bitmap at 655360 (+0), Inode bitmap at 655361 (+1)
>  Inode table at 655362-655873 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 21: (Blocks 688128-720895) [ITABLE_ZEROED]
>  Checksum 0x408d, unused inodes 0
>  Block bitmap at 688128 (+0), Inode bitmap at 688129 (+1)
>  Inode table at 688130-688641 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 22: (Blocks 720896-753663) [ITABLE_ZEROED]
>  Checksum 0x31de, unused inodes 0
>  Block bitmap at 720896 (+0), Inode bitmap at 720897 (+1)
>  Inode table at 720898-721409 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 23: (Blocks 753664-786431) [ITABLE_ZEROED]
>  Checksum 0x1eef, unused inodes 0
>  Block bitmap at 753664 (+0), Inode bitmap at 753665 (+1)
>  Inode table at 753666-754177 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 24: (Blocks 786432-819199) [ITABLE_ZEROED]
>  Checksum 0xeaf3, unused inodes 0
>  Block bitmap at 786432 (+0), Inode bitmap at 786433 (+1)
>  Inode table at 786434-786945 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 25: (Blocks 819200-851967) [ITABLE_ZEROED]
>  Checksum 0x839c, unused inodes 0
>  Backup superblock at 819200, Group descriptors at 819201-819201
>  Reserved GDT blocks at 819202-819264
>  Block bitmap at 819265 (+65), Inode bitmap at 819266 (+66)
>  Inode table at 819267-819778 (+67)
>  32189 free blocks, 8192 free inodes, 0 directories
> Group 26: (Blocks 851968-884735) [ITABLE_ZEROED]
>  Checksum 0xb491, unused inodes 0
>  Block bitmap at 851968 (+0), Inode bitmap at 851969 (+1)
>  Inode table at 851970-852481 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 27: (Blocks 884736-917503) [ITABLE_ZEROED]
>  Checksum 0xddfe, unused inodes 0
>  Backup superblock at 884736, Group descriptors at 884737-884737
>  Reserved GDT blocks at 884738-884800
>  Block bitmap at 884801 (+65), Inode bitmap at 884802 (+66)
>  Inode table at 884803-885314 (+67)
>  32189 free blocks, 8192 free inodes, 0 directories
> Group 28: (Blocks 917504-950271) [ITABLE_ZEROED]
>  Checksum 0x5637, unused inodes 0
>  Block bitmap at 917504 (+0), Inode bitmap at 917505 (+1)
>  Inode table at 917506-918017 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 29: (Blocks 950272-983039) [ITABLE_ZEROED]
>  Checksum 0x7906, unused inodes 0
>  Block bitmap at 950272 (+0), Inode bitmap at 950273 (+1)
>  Inode table at 950274-950785 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 30: (Blocks 983040-1015807) [ITABLE_ZEROED]
>  Checksum 0x0855, unused inodes 0
>  Block bitmap at 983040 (+0), Inode bitmap at 983041 (+1)
>  Inode table at 983042-983553 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
> Group 31: (Blocks 1015808-1048575) [ITABLE_ZEROED]
>  Checksum 0x2764, unused inodes 0
>  Block bitmap at 1015808 (+0), Inode bitmap at 1015809 (+1)
>  Inode table at 1015810-1016321 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories
>
> dumpe2fs: a.img: error reading bitmaps: Can't read an block bitmap
>
> $ truncate -s 4G a.img
>
> $ e2fsck -f a.img
> e2fsck 1.42-WIP (02-Jul-2011)
> Backing up journal inode block information.
>
> Moving journal from /.journal to hidden inode.
>
> Pass 1: Checking inodes, blocks, and sizes
> Pass 2: Checking directory structure
> Pass 3: Checking directory connectivity
> Pass 4: Checking reference counts
> Pass 5: Checking group summary information
> Block bitmap differences:  +(262144--262657) +(294912--295490) +(327680--328193) +(360448--360961) +(393216--393729) +(425984--426497) +(458752--459265) +(491520--492033) +(524288--524801) +(557056--557569) +(589824--590337) +(622592--623105) +(655360--655873) +(688128--688641) +(720896--721409) +(753664--754177) +(786432--786945) +(819200--819778) +(851968--852481) +(884736--885314) +(917504--918017) +(950272--950785) +(983040--983553) +(1015808--1016321)
> Fix<y>? yes
>
>
> a.img: ***** FILE SYSTEM WAS MODIFIED *****
> a.img: 20864/262144 files (0.0% non-contiguous), 73060/1048576 blocks
>
> $ dumpe2fs a.img
> dumpe2fs 1.42-WIP (02-Jul-2011)
> Filesystem volume name:   <none>
> Last mounted on:          /mnt
> Filesystem UUID:          f5a32b70-423d-448a-9550-90d67ff8b247
> Filesystem magic number:  0xEF53
> Filesystem revision #:    1 (dynamic)
> Filesystem features:      has_journal ext_attr resize_inode dir_index filetype extent flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize
> Filesystem flags:         signed_directory_hash
> Default mount options:    user_xattr acl
> Filesystem state:         clean
> Errors behavior:          Continue
> Filesystem OS type:       Linux
> Inode count:              262144
> Block count:              1048576
> Reserved block count:     52419
> Free blocks:              975516
> Free inodes:              241280
> First block:              0
> Block size:               4096
> Fragment size:            4096
> Reserved GDT blocks:      63
> Blocks per group:         32768
> Fragments per group:      32768
> Inodes per group:         8192
> Inode blocks per group:   512
> Flex block group size:    32
> Filesystem created:       Tue Aug  2 14:55:16 2011
> Last mount time:          Tue Aug  2 14:55:24 2011
> Last write time:          Tue Aug  2 15:33:44 2011
> Mount count:              0
> Maximum mount count:      20
> Last checked:             Tue Aug  2 15:33:44 2011
> Check interval:           0 (<none>)
> Lifetime writes:          566 kB
> Reserved blocks uid:      0 (user root)
> Reserved blocks gid:      0 (group root)
> First inode:              11
> Inode size:               256
> Required extra isize:     28
> Desired extra isize:      28
> Journal inode:            8
> Default directory hash:   half_md4
> Directory Hash Seed:      6aa182f8-2ce9-4b90-9138-51b2fcf09ff0
> Journal backup:           inode blocks
> Journal features:         (none)
> Journal size:             128M
> Journal length:           32768
> Journal sequence:         0x00000001
> Journal start:            0
>
>
> Group 0: (Blocks 0-32767) [ITABLE_ZEROED]
>  Checksum 0x6170, unused inodes 0
>  Primary superblock at 0, Group descriptors at 1-1
>  Reserved GDT blocks at 2-64
>  Block bitmap at 65 (+65), Inode bitmap at 97 (+97)
>  Inode table at 129-640 (+129)
>  24746 free blocks, 0 free inodes, 3 directories
>  Free blocks: 8022-32767
>  Free inodes:
> Group 1: (Blocks 32768-65535) [ITABLE_ZEROED]
>  Checksum 0x4df3, unused inodes 0
>  Backup superblock at 32768, Group descriptors at 32769-32769
>  Reserved GDT blocks at 32770-32832
>  Block bitmap at 66 (bg #0 + 66), Inode bitmap at 98 (bg #0 + 98)
>  Inode table at 641-1152 (bg #0 + 641)
>  9128 free blocks, 0 free inodes, 975 directories
>  Free blocks: 49152-57343, 60376-60383, 60512-61439
>  Free inodes:
> Group 2: (Blocks 65536-98303) [ITABLE_ZEROED]
>  Checksum 0x7bcd, unused inodes 4077
>  Block bitmap at 67 (bg #0 + 67), Inode bitmap at 99 (bg #0 + 99)
>  Inode table at 1153-1664 (bg #0 + 1153)
>  4096 free blocks, 4077 free inodes, 2302 directories, 4077 unused inodes
>  Free blocks: 94208-98303
>  Free inodes: 20500-24576
> Group 3: (Blocks 98304-131071) [ITABLE_ZEROED]
>  Checksum 0x69b9, unused inodes 7865
>  Backup superblock at 98304, Group descriptors at 98305-98305
>  Reserved GDT blocks at 98306-98368
>  Block bitmap at 68 (bg #0 + 68), Inode bitmap at 100 (bg #0 + 100)
>  Inode table at 1665-2176 (bg #0 + 1665)
>  32703 free blocks, 7865 free inodes, 327 directories, 7865 unused inodes
>  Free blocks: 98369-131071
>  Free inodes: 24904-32768
> Group 4: (Blocks 131072-163839) [ITABLE_ZEROED]
>  Checksum 0x12e8, unused inodes 8154
>  Block bitmap at 69 (bg #0 + 69), Inode bitmap at 101 (bg #0 + 101)
>  Inode table at 2177-2688 (bg #0 + 2177)
>  32768 free blocks, 8154 free inodes, 38 directories, 8154 unused inodes
>  Free blocks: 131072-163839
>  Free inodes: 32807-40960
> Group 5: (Blocks 163840-196607) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0xe3c3, unused inodes 8192
>  Backup superblock at 163840, Group descriptors at 163841-163841
>  Reserved GDT blocks at 163842-163904
>  Block bitmap at 70 (bg #0 + 70), Inode bitmap at 102 (bg #0 + 102)
>  Inode table at 2689-3200 (bg #0 + 2689)
>  32703 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 163905-196607
>  Free inodes: 40961-49152
> Group 6: (Blocks 196608-229375) [INODE_UNINIT, BLOCK_UNINIT, ITABLE_ZEROED]
>  Checksum 0xaac1, unused inodes 8192
>  Block bitmap at 71 (bg #0 + 71), Inode bitmap at 103 (bg #0 + 103)
>  Inode table at 3201-3712 (bg #0 + 3201)
>  32768 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 196608-229375
>  Free inodes: 49153-57344
> Group 7: (Blocks 229376-262143) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0xa250, unused inodes 8192
>  Backup superblock at 229376, Group descriptors at 229377-229377
>  Reserved GDT blocks at 229378-229440
>  Block bitmap at 72 (bg #0 + 72), Inode bitmap at 104 (bg #0 + 104)
>  Inode table at 3713-4224 (bg #0 + 3713)
>  32703 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 229441-262143
>  Free inodes: 57345-65536
> Group 8: (Blocks 262144-294911) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0x8219, unused inodes 8192
>  Block bitmap at 262144 (+0), Inode bitmap at 262145 (+1)
>  Inode table at 262146-262657 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 262658-294911
>  Free inodes: 65537-73728
> Group 9: (Blocks 294912-327679) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0xeb76, unused inodes 8192
>  Backup superblock at 294912, Group descriptors at 294913-294913
>  Reserved GDT blocks at 294914-294976
>  Block bitmap at 294977 (+65), Inode bitmap at 294978 (+66)
>  Inode table at 294979-295490 (+67)
>  32189 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 295491-327679
>  Free inodes: 73729-81920
> Group 10: (Blocks 327680-360447) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0xdc7b, unused inodes 8192
>  Block bitmap at 327680 (+0), Inode bitmap at 327681 (+1)
>  Inode table at 327682-328193 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 328194-360447
>  Free inodes: 81921-90112
> Group 11: (Blocks 360448-393215) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0xf34a, unused inodes 8192
>  Block bitmap at 360448 (+0), Inode bitmap at 360449 (+1)
>  Inode table at 360450-360961 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 360962-393215
>  Free inodes: 90113-98304
> Group 12: (Blocks 393216-425983) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0x3edd, unused inodes 8192
>  Block bitmap at 393216 (+0), Inode bitmap at 393217 (+1)
>  Inode table at 393218-393729 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 393730-425983
>  Free inodes: 98305-106496
> Group 13: (Blocks 425984-458751) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0x11ec, unused inodes 8192
>  Block bitmap at 425984 (+0), Inode bitmap at 425985 (+1)
>  Inode table at 425986-426497 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 426498-458751
>  Free inodes: 106497-114688
> Group 14: (Blocks 458752-491519) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0x60bf, unused inodes 8192
>  Block bitmap at 458752 (+0), Inode bitmap at 458753 (+1)
>  Inode table at 458754-459265 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 459266-491519
>  Free inodes: 114689-122880
> Group 15: (Blocks 491520-524287) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0x4f8e, unused inodes 8192
>  Block bitmap at 491520 (+0), Inode bitmap at 491521 (+1)
>  Inode table at 491522-492033 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 492034-524287
>  Free inodes: 122881-131072
> Group 16: (Blocks 524288-557055) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0xc884, unused inodes 8192
>  Block bitmap at 524288 (+0), Inode bitmap at 524289 (+1)
>  Inode table at 524290-524801 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 524802-557055
>  Free inodes: 131073-139264
> Group 17: (Blocks 557056-589823) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0xe7b5, unused inodes 8192
>  Block bitmap at 557056 (+0), Inode bitmap at 557057 (+1)
>  Inode table at 557058-557569 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 557570-589823
>  Free inodes: 139265-147456
> Group 18: (Blocks 589824-622591) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0x96e6, unused inodes 8192
>  Block bitmap at 589824 (+0), Inode bitmap at 589825 (+1)
>  Inode table at 589826-590337 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 590338-622591
>  Free inodes: 147457-155648
> Group 19: (Blocks 622592-655359) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0xb9d7, unused inodes 8192
>  Block bitmap at 622592 (+0), Inode bitmap at 622593 (+1)
>  Inode table at 622594-623105 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 623106-655359
>  Free inodes: 155649-163840
> Group 20: (Blocks 655360-688127) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0x7440, unused inodes 8192
>  Block bitmap at 655360 (+0), Inode bitmap at 655361 (+1)
>  Inode table at 655362-655873 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 655874-688127
>  Free inodes: 163841-172032
> Group 21: (Blocks 688128-720895) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0x5b71, unused inodes 8192
>  Block bitmap at 688128 (+0), Inode bitmap at 688129 (+1)
>  Inode table at 688130-688641 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 688642-720895
>  Free inodes: 172033-180224
> Group 22: (Blocks 720896-753663) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0x2a22, unused inodes 8192
>  Block bitmap at 720896 (+0), Inode bitmap at 720897 (+1)
>  Inode table at 720898-721409 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 721410-753663
>  Free inodes: 180225-188416
> Group 23: (Blocks 753664-786431) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0x0513, unused inodes 8192
>  Block bitmap at 753664 (+0), Inode bitmap at 753665 (+1)
>  Inode table at 753666-754177 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 754178-786431
>  Free inodes: 188417-196608
> Group 24: (Blocks 786432-819199) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0xf10f, unused inodes 8192
>  Block bitmap at 786432 (+0), Inode bitmap at 786433 (+1)
>  Inode table at 786434-786945 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 786946-819199
>  Free inodes: 196609-204800
> Group 25: (Blocks 819200-851967) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0x9860, unused inodes 8192
>  Backup superblock at 819200, Group descriptors at 819201-819201
>  Reserved GDT blocks at 819202-819264
>  Block bitmap at 819265 (+65), Inode bitmap at 819266 (+66)
>  Inode table at 819267-819778 (+67)
>  32189 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 819779-851967
>  Free inodes: 204801-212992
> Group 26: (Blocks 851968-884735) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0xaf6d, unused inodes 8192
>  Block bitmap at 851968 (+0), Inode bitmap at 851969 (+1)
>  Inode table at 851970-852481 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 852482-884735
>  Free inodes: 212993-221184
> Group 27: (Blocks 884736-917503) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0xc602, unused inodes 8192
>  Backup superblock at 884736, Group descriptors at 884737-884737
>  Reserved GDT blocks at 884738-884800
>  Block bitmap at 884801 (+65), Inode bitmap at 884802 (+66)
>  Inode table at 884803-885314 (+67)
>  32189 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 885315-917503
>  Free inodes: 221185-229376
> Group 28: (Blocks 917504-950271) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0x4dcb, unused inodes 8192
>  Block bitmap at 917504 (+0), Inode bitmap at 917505 (+1)
>  Inode table at 917506-918017 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 918018-950271
>  Free inodes: 229377-237568
> Group 29: (Blocks 950272-983039) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0x62fa, unused inodes 8192
>  Block bitmap at 950272 (+0), Inode bitmap at 950273 (+1)
>  Inode table at 950274-950785 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 950786-983039
>  Free inodes: 237569-245760
> Group 30: (Blocks 983040-1015807) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0x13a9, unused inodes 8192
>  Block bitmap at 983040 (+0), Inode bitmap at 983041 (+1)
>  Inode table at 983042-983553 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 983554-1015807
>  Free inodes: 245761-253952
> Group 31: (Blocks 1015808-1048575) [INODE_UNINIT, ITABLE_ZEROED]
>  Checksum 0x3c98, unused inodes 8192
>  Block bitmap at 1015808 (+0), Inode bitmap at 1015809 (+1)
>  Inode table at 1015810-1016321 (+2)
>  32254 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
>  Free blocks: 1016322-1048575
>  Free inodes: 253953-262144
>
> $ sudo mount -t ext4 -o loop a.img /mnt/
>
> $ df
> Filesystem           1K-blocks      Used Available Use% Mounted on
> /dev/loop0             4128448    226384   3692388   6% /mnt
>
> $ sudo umount /mnt
> --------------------------------
> --------------------------------
> --------------------------------
>
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