Linux-kernel mailing list archive 1999-23,: mount union

# mount union

Thomas Pornin (pornin@bolet.ens.fr)
Sat, 12 Jun 1999 10:43:27 +0200

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Hello,

I don't think this issue has been recently discussed, but if what I ask
is stupid, please do not shoot me.

BSD has an option called 'union' that makes it somehow merge a mounted
filesystem with the files that already exist: files are created on the
newly mounted filesystem, but the underlying files may be accessed if no
file with the same name exists on the new filesystem.

I really like this because it is really fun, and, moreover, it might
as well be useful, for instance for:
-- diskless machines: /etc NFS mounted, same for every one, with a
very small ramdisk mounted over it, that contains the machine-specific
information
-- compiling from a cdrom with sources (writes are done on a disk), or
from a read-only partition
-- sharing /usr/local across different architectures, with only
binaries that differ (data files, shell scripts, perl scripts are
identical, therefore not copied)(sounds like Plan9)

Is this handled by some patch somewhere ? Or are some people working on
it ? Is it feasible easily at least ?

Another related fun feature: one filesystem read-only, another one
mounted read/write over it. When you open a file for writing, if it is
on the read-only filesystem, it is first copied on the read/write one.
It would be some sort of patching filesystem.

Ok, maybe all of this is just lunatic ranting.

--Thomas Pornin

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