Re: Union file system

David McIntyre (
Wed, 27 Nov 1996 15:55:17 -0800 (PST)

On Wed, 27 Nov 1996, D. Chiodo wrote:

> On Wed, 27 Nov 1996, Keith Owens wrote:
> > Does anybody know of a union file system for Linux? Alternatively any
> > writeups on why it is/is not a good idea?
> >
> > Looking for a way to take multiple levels of data, some on cdrom, some on
> > network, some on disk and make it look like a single level. All updates
> > should be written to the top level.
> Symlinks? Mount all the cd, network, disk, etc, then make symlinks from
> whereyou want you everything to appear to wherever the appropriate files
> are...

Symlinks have the disadvantage of being "seperate" filesystems. The idea
of a union filesystem is to make all drive space on all drives appear as
a contiguous block. Therefor, it would be harder to run out of space on
one of the component filesystems. For example, if you mount /home on a
1gig drive and you have 900meg used on it, you would be unable to untar a
200meg file, even if you had 3gig on the / partition. A union filesystem
would show that you had the available free space of all free space on all
physical devices. This is possible to do in Novell with a base system.
It has a couple of quirks which only show up in unusual situations. In
Novell, you can't split a single file across physical devices. It just
begins on the next device if a file is about to overrun one device. This
is usually not a factor unless you are working with a HUGE file, like a
big database. But then, if this were our application, we'd buy a drive
large enough to support the file, wouldn't we? :)

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