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Steven Oxley
Asked: 2022-10-05 19:33:52 +0800 CST

Why is a filesystem created with mkfs not available immediately

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Why are file systems created with mkfs not immediately available after creation?

The following command line probably best describes what I'm seeing:

/sbin/mkfs.ext4 /dev/sdc && lsblk -o uuid --noheadings /dev/sdc

First I'm creating an ext4 filesystem on /dev/sdc and then I'm trying to immediately show the UUID of that filesystem.

This is the corresponding output of those commands:

mke2fs 1.44.1 (24-Mar-2018)
/dev/sdc contains a ext4 file system
        created on Wed Oct  5 00:40:08 2022
Proceed anyway? (y,N) y
Discarding device blocks: done
Creating filesystem with 268435456 4k blocks and 67108864 inodes
Filesystem UUID: d54169aa-faab-49b5-b0fd-fdf98b603df0
Superblock backups stored on blocks:
        32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
        4096000, 7962624, 11239424, 20480000, 23887872, 71663616, 78675968,
        102400000, 214990848

Allocating group tables: done
Writing inode tables: done
Creating journal (262144 blocks): done
Writing superblocks and filesystem accounting information: done

434269b5-59e4-4fd0-95f0-376090e0e0a9

You'll notice that "Filesystem UUID" from the mkfs.ext4 output does not match the output of lsblk shown at the end of the output.

However, if I rerun lsblk -o uuid --noheadings /dev/sdc a little bit later, I get:

d54169aa-faab-49b5-b0fd-fdf98b603df0

And that does match the mkfs.ext4 output. Why the delay?

linux

1 Answers

  1. If mkfs.ext4 returns without error, it can be used, including by file system UUID. mount UUID=434269b5-59e4-4fd0-95f0-376090e0e0a9 /mnt or whatever, and put it in fstab if you are happy.

    On Linux, some distros decided a nice hardware experience means some user space programs meddle in the process. Namely udev (hardware events system) and blkid (block device probing functions). Per man lsblk:

    Note that lsblk might be executed in time when udev does not have all information about recently added or modified devices yet. In this case it is recommended to use udevadm settle before lsblk to synchronize with udev.

    A user space program created a file system, and a user space program figured out the resulting UUID, but they have no direct integration. Also lsblk has a cache. Resulting in short delays. And stale data from any previous file system, note that you were re-creating over an existing ext4 without something like a wipefs command.

    Mounting it directly results in a low level blkid probe same as the command blkid --uuid d54169aa-faab-49b5-b0fd-fdf98b603df0 which by default will scan all block devices to find unknown volumes.

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