Recover DD-d partition info

hunandoz

Dear all,

I have unfortunately deleted my sda partition info with (dd if=/dev/zero of=/dev/sda) when I wanted to repair my SD Card partion table in sdb. :-( 
Right now system is up and running, but if I lose power, perform restart -->everything will be gone. I do not have backup possibilty of the total NAS content.
If possible I would like to recover the partitions without removing the HDD from the system. Also if possible I do not want to install debian/ubuntu onto a PC.

Thank you for your help in advance!

lsblk:

NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT

loop0 7:0 0 140M 1 loop /ram_bin

sda 8:0 0 1.8T 0 disk:

|-sda1 8:1 0 502M 0 part /zyxel/mnt/sysdisk

`-sda2 8:2 0 1.8T 0 part

`-md0 9:0 0 1.8T 0 linear /i-data/1877af91

fdisk -l:

Disk /dev/sda: 2000.4 GB, 2000398934016 bytes

255 heads, 63 sectors/track, 243201 cylinders, total 3907029168 sectors

Units = sectors of 1 * 512 = 512 bytes

Sector size (logical/physical): 512 bytes / 512 bytes

I/O size (minimum/optimal): 512 bytes / 512 bytes

Disk identifier: 0x00000000

Disk /dev/sda doesn't contain a valid partition table


#NAS_Feb_2020

hunandoz

Dear All,
if mdstat states md0 has active device  sda2 - am I ok?
cat /proc/mdstat

Personalities : [linear] [raid0] [raid1]

md0 : active linear sda2[0]

      1952996928 blocks super 1.2 0k rounding

mdadm --examine /dev/sda1

mdadm: No md superblock detected on /dev/sda1.

mdadm --examine /dev/sda2

/dev/sda2:

Magic : a92b4efc

Version : 1.2

Feature Map : 0x0

Array UUID : 1877af91:c4d12aaf:62318110:85a049b4

Name : NAS:0 (local to host NAS)

Creation Time : Sun Mar 24 23:38:55 2019

Raid Level : linear

Raid Devices : 1

Avail Dev Size : 1952996928 (1862.52 GiB 1999.87 GB)

Used Dev Size : 0

Data Offset : 2048 sectors

Super Offset : 8 sectors

State : clean

Device UUID : 2a4ec1ab:c15d1211:dbf7ffc6:10f1e4d3

Update Time : Sun Mar 24 23:38:55 2019

Checksum : e2dc1423 - correct

Events : 0

Rounding : 0K

Device Role : Active device 0

Array State : A ('A' == active, '.' == missing)

Mijzelf

You'll have to recreate the partition table manually. You can find the start sector and size of the partitions with

cat /sys/block/sda/sda1/start

cat /sys/block/sda/sda1/size

cat /sys/block/sda/sda2/start

cat /sys/block/sda/sda2/size

hunandoz

Mijzelf,
Thank you for the starting step; Now I know the start/size for both partition. 
cat /sys/block/sda/sda1/start

63

cat /sys/block/sda/sda1/size

1028097

cat /sys/block/sda/sda2/start

1028160

cat /sys/block/sda/sda2/size

3905995905

Can ou help me with fdisk as well?
I am afraid as the NAS sees the volume (sda2) as JBOD [as far as I understand it], so I do not want crush it.

hunandoz

I have just checked one more thing; /dev/md0 is as well existing in fdisk. 

fdisk -l

Disk /dev/sda: 2000.4 GB, 2000398934016 bytes

255 heads, 63 sectors/track, 243201 cylinders, total 3907029168 sectors

Units = sectors of 1 * 512 = 512 bytes

Sector size (logical/physical): 512 bytes / 512 bytes

I/O size (minimum/optimal): 512 bytes / 512 bytes

Disk identifier: 0x00000000

Disk /dev/sda doesn't contain a valid partition table

Disk /dev/md0: 1999.9 GB, 1999868854784 bytes

2 heads, 4 sectors/track, 488249232 cylinders, total 3905993857 sectors

Units = sectors of 1 * 512 = 512 bytes

Sector size (logical/physical): 512 bytes / 512 bytes

I/O size (minimum/optimal): 512 bytes / 512 bytes

Disk identifier: 0x00000000

Disk /dev/md0 doesn't contain a valid partition table

hunandoz

Mijzelf,

As far as I have checked;
fdisk /dev/sda (am I need sudo?)
select "n" as new partition
select "p" as primary (? - it will "name" it as sda1?)
enter "63"
enter "1028097"
repeat with sda2 data
n, p, 1028160, 3905995905.

How to connect it to md0? (Sorry for my poor linux knowledge)
Thank you for your patience, and understanding,

Mijzelf

I have just checked one more thing; /dev/md0 is as well existing in fdisk.

fdisk runs on any block device, and even on files. But as you can see, md0 doesn't contain a valid partition table, so it's pretty meaningless here.

As far as I have checked;
fdisk /dev/sda (am I need sudo?)

By default there is no sudo on the box. If you logged in using the same credentials as you used to dd the partition table away, you don't need it.

(? - it will "name" it as sda1?)

Yes.

How to connect it to md0?

You don't need to. The partition table is not more then a database which contains the location and size of the partitions. You didn't change anything in the partition itself, so if you recover the table, the partitions will be as they always were. sda2 contains a header which tells it's a part of md0.

hunandoz

Mijzelf,
Thank you for the given instruction. 
But I was unable to create 1st partition, as the possible value range is 2048  - 3907029167.
I have stopped / quit from fdisk.

hunandoz

Mijzelf,
One more question. I am able to create the sda2, the id is 83 - "linux" is it OK?

hunandoz

Mijzelf, 
Sorry to asking many questions. 
The end sector shall be 1028160 + 3905995905 = 3907029167
Am I right?

hunandoz

I have googled, and switched on "dos compatibility flag" with issue "c". Many thanks!!!
Shall I do anything else?