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What do default_mountopts do?
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Bones McCracker
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PostPosted: Wed Apr 17, 2013 4:47 pm    Post subject: What do default_mountopts do? Reply with quote

The ext series of filesystems include in the superblock a field containing "default mount options". These are based on what you have defined in /etc/mke2fs.conf, and can be set on a per-filesystem basis using tune2fs.

By default, the mke2fs.conf file comes with these set to be:
Code:
default_mntopts = acl,user_xattr


You can confirm that these exist in the created filesystem by doing 'tune2fs -l', which will show:
Code:
Default mount options:    user_xattr acl


However, when the filesystem is mounted, either by fstab (i.e., 'mount -a'), or a manual 'mount /dev/foo /mnt/bar' command, these defaults are apparently ignored. In the case of fstab, whatever options are present there are used. In the case of a basic 'mount' command (with no options specified), they are also ignored.

So what purpose do these "default mount options" serve? How are they used?
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Bones McCracker
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PostPosted: Fri Apr 19, 2013 12:29 am    Post subject: Reply with quote

Bump. Hello, asswipes? Anybody with a clue out there?
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pjp
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PostPosted: Fri Apr 19, 2013 2:13 am    Post subject: Reply with quote

Not much information out there on that one. I wonder if it's a bug.
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Bones McCracker
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PostPosted: Fri Apr 19, 2013 2:27 am    Post subject: Reply with quote

One must imagine that 'mount' once paid attention to it. Probably just one of many vestigial bits of cruft that nobody knows anything about and is afraid to get rid of because it might break something. And now it's part of ext4. :lol:
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pjp
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PostPosted: Fri Apr 19, 2013 3:09 am    Post subject: Reply with quote

Almost seems like it shouldn't be horrendously difficult to find in the code.
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Bones McCracker
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PostPosted: Fri Apr 19, 2013 3:05 pm    Post subject: Reply with quote

This reminds me of something else. One of the man pages for something (I can't remember what but think it was X-related) has an option documented to the effect of "Nobody anywhere seems to know what this does." :lol:
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PostPosted: Fri Apr 19, 2013 3:36 pm    Post subject: Reply with quote

BoneKracker wrote:
This reminds me of something else. One of the man pages for something (I can't remember what but think it was X-related) has an option documented to the effect of "Nobody anywhere seems to know what this does." :lol:

Ya gotta love Linux. :)
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PaulBredbury
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PostPosted: Fri Apr 19, 2013 4:53 pm    Post subject: Reply with quote

In util-linux, Documentation/releases/v2.21-ReleaseNotes contains:
Quote:
Changes between v2.20 and v2.21
...
libmount
- ignore "defaults" mount option [Karel Zak]

And in mount-deprecated/mount.c:
Code:
  { "defaults", 0, 0, 0     },  /* default options */


Although I don't see a reason *why*.
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pjp
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PostPosted: Fri Apr 19, 2013 10:26 pm    Post subject: Reply with quote

BoneKracker wrote:
This reminds me of something else. One of the man pages for something (I can't remember what but think it was X-related) has an option documented to the effect of "Nobody anywhere seems to know what this does." :lol:
In those cases it seems a perfectly good idea to comment out the code on a development cycle and see what happens.
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Bones McCracker
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PostPosted: Sat Apr 20, 2013 1:04 am    Post subject: Reply with quote

PaulBredbury wrote:
In util-linux, Documentation/releases/v2.21-ReleaseNotes contains:
Quote:
Changes between v2.20 and v2.21
...
libmount
- ignore "defaults" mount option [Karel Zak]

And in mount-deprecated/mount.c:
Code:
  { "defaults", 0, 0, 0     },  /* default options */


Although I don't see a reason *why*.

Thank you. You are a gentleman and a scholar.
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