location of file-system information on ext4

[Martin T]

Hi,

I zero-filled first 10MiB of my SSD(dd if=/dev/zero of=/dev/sda bs=10M
count=1). As expected, this wiped my primary GPD header and first
partition. Before the wipe, GPT was following:

Disk /dev/sda: 250069680 sectors, 119.2 GiB
Logical sector size: 512 bytes
Disk identifier (GUID): 2EFD285D-F8E6-4262-B380-232E866AF15C
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 250069646
Partitions will be aligned on 1-sector boundaries
Total free space is 16 sectors (8.0 KiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1              34        58593784   27.9 GiB    EF00  root part
   2        58593785       234375035   83.8 GiB    0700  home part
   3       234375036       250069630   7.5 GiB     8200  swap part


However, to my surprise, the file(1) and dumpe2fs(8) utilities were
still able to show information regarding file system:

root at T60:~# file -s /dev/sda1
/dev/sda1: sticky Linux rev 1.0 ext4 filesystem data,
UUID=88e70e1b-4c45-4e7e-931f-9e97d7257ee8 (needs journal recovery)
(extents) (large files) (huge files)
root at T60:~# dumpe2fs /dev/sda1
dumpe2fs 1.42.5 (29-Jul-2012)
Filesystem volume name:   <none>
Last mounted on:          /
Filesystem UUID:          88e70e1b-4c45-4e7e-931f-9e97d7257ee8
Filesystem magic number:  0xEF53
Filesystem revision #:    1 (dynamic)
Filesystem features:      has_journal ext_attr resize_inode dir_index
filetype needs_recovery extent flex_bg sparse_super large_file
huge_file uninit_bg dir_nlink extra_isize
Filesystem flags:         signed_directory_hash
Default mount options:    (none)
Filesystem state:         clean
Errors behavior:          Continue
Filesystem OS type:       Linux
Inode count:              1831424
Block count:              7324218
Reserved block count:     366210
Free blocks:              4511822
Free inodes:              1671701
First block:              0
Block size:               4096
Fragment size:            4096
Reserved GDT blocks:      1022
Blocks per group:         32768
Fragments per group:      32768
Inodes per group:         8176
Inode blocks per group:   511
Flex block group size:    16
Filesystem created:       Wed Jul 10 23:01:50 2013
Last mount time:          Mon May  5 19:28:01 2014
Last write time:          Mon May  5 19:28:01 2014
Mount count:              14
Maximum mount count:      34
Last checked:             Sun Jan 26 15:32:00 2014
Check interval:           15552000 (6 months)
Next check after:         Fri Jul 25 15:32:00 2014
Lifetime writes:          61 GB
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
First orphan inode:       1439005
Default directory hash:   half_md4
Directory Hash Seed:      f7476ad6-3067-4697-93b3-f4a368d76eab
Journal backup:           inode blocks
Journal superblock magic number invalid!
root at T60:~#


Am I correct that there are multiple super-blocks scattered around the
HDD and if first one is missing, then next one is read by file or
dumpe2fs utilities? In the first place, what (important) file-system
data is written at the beginning of the partition?



regards,
Martin