Having losetup read the partition table [closed] - linux

Question

Closed. This question does not meet Stack Overflow guidelines. It is not currently accepting answers.
This question does not appear to be about a specific programming problem, a software algorithm, or software tools primarily used by programmers. If you believe the question would be on-topic on another Stack Exchange site, you can leave a comment to explain where the question may be able to be answered.
Closed 6 years ago.
Improve this question
For the purpose of learning, I wanted to create a mini-replica of my hard disk:
$ lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 931.5G 0 disk
├─sda1 8:1 0 512M 0 part /boot/efi
├─sda2 8:2 0 244M 0 part /boot
└─sda3 8:3 0 930.8G 0 part
└─sda3_crypt 254:0 0 930.8G 0 crypt
├─host--vg-root 254:1 0 25G 0 lvm /
├─host--vg-var 254:2 0 2.8G 0 lvm /var
├─host--vg-swap_1 254:3 0 11.9G 0 lvm [SWAP]
├─host--vg-tmp 254:4 0 380M 0 lvm /tmp
└─host--vg-home 254:5 0 890G 0 lvm /home
In my particular case, setting up a new device so it looks like my hard disk requires familiarity with many things, creating partitions, creating LUKS devices, opening them, creating LVM volumes etc, so I regard this as a worthy exercise, at least for someone who is new to Linux.
So I first needed a new device to play with, without messing up anything else:
$ dd if=/dev/zero of=loopfile bs=1M count=1024
$ sudo losetup /dev/loop1 loopfile
(using loop1 rather than loop0 which is already taken for some other purpose, also zero good enough for this exercise so ignoring urandom).
My first objective was to mimic the partitions sda1/sda2/sda3
$ sudo blkid
/dev/sda1: UUID="08FC-EA23" TYPE="vfat" ...
/dev/sda2: UUID="30b5d595-4986-4f75-962a-7e1f5f03ed4a" TYPE="ext2" ...
/dev/sda3: UUID="a84cc598-9316-48b9-94a9-bb4885e45e9c" TYPE="crypto_LUKS" ...
$ sudo parted /dev/loop1
So I went and created three 'primary' partitions (using 'fat32' for the first one and 'ext2' for other two, not too sure why just guessing) with all sizes reduced by a factor of 1000:
(parted) print
Number Start End Size Type File system Flags
1 512B 1000kB 1000kB primary fat32 lba
2 1049kB 2097kB 1049kB primary ext2 lba
3 2097kB 1074MB 1072MB primary ext2 lba
and I then formatted the three devices in line with the previous blkid report:
sudo mkfs -t vfat /dev/loop1p1
sudo mkfs -t ext2 /dev/loop1p2
sudo cryptsetup luksFormat /dev/loop1p3
So at this point, my parted print report looks good as well as lsblk and blkid:
$ lsblk
loop1 7:1 0 1G 0 loop
├─loop1p1 259:0 0 976.5K 0 loop
├─loop1p2 259:1 0 1M 0 loop
└─loop1p3 259:2 0 1022M 0 loop
$ sudo blkid
/dev/loop1p1: SEC_TYPE="msdos" UUID="1CD8-2CA5" TYPE="vfat" ...
/dev/loop1p2: UUID="6532dba9-3101-488e-a6d1-e5e1ef4943f7" TYPE="ext2" ...
/dev/loop1p3: UUID="a0e96a54-6d6a-49c8-80fd-03217b25062f" TYPE="crypto_LUKS" ...
/dev/loop1: PTUUID="1de285f7" PTTYPE="dos"
So I thought I was on the right track. I also thought that my file loopfile which underlies my loop device would contain of the necessary metadata so I do not need to worry about rebooting. As I am only playing with devices (not mounting them) I assumed there is no need for any /etc/fstab setup...
The issue I have is that when I reboot, some of the set up seems to be lost. After re-creating the loop device from loopfile, the parted print report still shows me the partitions (albeit with lost information on type), but these partitions no longer appear on the lsblk or the blkid reports. I was wondering if there was a way of making my set up persistant. I am on Debian 8, in case this matters.

Answer 1

You need to run losetup -P /dev/loop1 loopfile. What this does is tell the kernel to perform a partition table scan of the newly added file.