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I would have expected an error, sorry FS is read-only, but it is possible. This is unexpected & counter intuitive is there a reason?

Linux files 5.11.0-27-generic #29~20.04.1-Ubuntu SMP Wed Aug 11 15:58:17 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
files@files:/mnt/disk$ mount | grep /mnt/disk/005
/dev/sdh on /mnt/disk/005 type ext4 (ro,nosuid,nodev,noexec,relatime,user)
files@files:/mnt/disk$ df /mnt/disk/005/
Filesystem      1K-blocks       Used Available Use% Mounted on
/dev/sdh       7751367424 7332824836  27824876 100% /mnt/disk/005
files@files:/mnt/disk$ sudo tune2fs -r 0 /dev/sdh
tune2fs 1.45.5 (07-Jan-2020)
Setting reserved blocks count to 0
files@files:/mnt/disk$ df /mnt/disk/005/
Filesystem      1K-blocks       Used Available Use% Mounted on
/dev/sdh       7751367424 7332824836 418526204  95% /mnt/disk/005
files@files:/mnt/disk$

1 Answer 1

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tune2fs never needed the filesystem to be mounted to operate. As you can see in the command, you pass a block device node (/dev/sdh) as an argument to it, not a mountpoint (/mnt/disk/005). It's similar to resize2fs and e2label.

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    While the read-only restriction on the mount has no effect here, Linux block devices could be marked as read-only in a similar fashion, e.g. blockdev --setro /dev/example restricts (future) access to the device and/or partition in question.
    – anx
    Sep 12, 2021 at 5:41
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    @anx AFAIK, that may not be something that is really reliable. (I've never really digged deep into how it works, but IIRC I've seen stories about it being unexpectedly "breakable".)
    – Tom Yan
    Sep 12, 2021 at 5:48
  • True. Propagation through partitions/md/rbd etc seems mostly reasonable these days, it just remains one of those little known, little used features that barely even get bugs reported.
    – anx
    Sep 12, 2021 at 6:03

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