Re: Union file system

Matthias Urlichs (
Thu, 28 Nov 1996 18:45:34 +0100 (MET)


Tuomas Heino wrote:
>> I don't really understand why loopback mounts don't exist in Linux then.
>> It really doesn't sound too hard to implement for someone who knows the fs
>> code of Linux fairly well.
>Well some kind of loopback mount exists AND is in the 2.*.* kernels ;)
>mount /tmp/fdimage /mnt -o loop=/dev/loop0,blocksize=1024

That is NOT what we all mean when we talk about a loopback mount.

A loopback mount means that, for example, /tmp is also visible as /var/tmp,
but _without_ a symlink from one to the other. Great when
/www/WHOEVER/bin and /lib needs to be a bunch of identical files for all
WHOEVERs, but cannot be symlinked because the servers chroot to

The "loop" device just makes a single file available as a fake block
device. A neat hack, but it's not what we need -- you can probably mount
the thing from several places at once, but you can't write to it.

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