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If ext4 partition is mounted with ro option:

/dev/sdxY    /    ext4    ro    0    1

are any additional options enabled implicitly too?

For instance, is any of the following options (which can affect on performance) enabled:
- noatime;
- barrier;
- data=writeback;
- nobh.

  • It depends on the kernel version. It's the same for ro and rw mounts. – Gilles Jul 28 '17 at 23:41
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Look at the corresponding entry in /proc/mounts, you will get the whole list of options added implicitly at mount time.

  • I've explored /proc/self/mount* and I didn't see that such options was enabled. But how can work atime in read-only mode? Or what is the purpose of barrier=1 (which is enabled by default on ext4) in read-only mode? – Gluttton Jul 28 '17 at 23:35
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    @Gluttton Write-mode options like atime or barrier have no effect on a read-only mount. But they will take effect as soon as you switch to read-write mode (mount -o remount,rw) – xhienne Jul 28 '17 at 23:57

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