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From: Jacques Gelinas (jack_at_solucorp.qc.ca)
Date: Mon 12 Aug 2002 - 20:26:30 BST

On Sat, 10 Aug 2002 10:30:28 -0500, ragnar_at_this.is wrote
> Hi,
> Is there a file system that does transparent unify?
> It does at least seem to belong in the file system.
> How much work to modify ext3?

One concept was proposed on linux kernel a while back. It is COW (Copy
On Write). This is currently widely used in kernel to handle memory. Applied to
file, it would mean that if you do

        cp a b

you end up with a and b both pointing to the same data. The operation does
not involve any copy at all. Mostly setting flags on metadata.

Then when you modify either file a or file b, new storage is allocated for
the modified chunks. a and b are still completly independant, yet they
share disk space at the block size level.

What is interesting here to us, vserver adict, is that it allows one to create
a completly unified vserver, including data (configuration files).

While doable, this creates a lot of burden for the fsck coder :-)

Note that SMB has this concept and this feature might enter the kernel
through smbfs. I don't remember too well, but their seemed to have
other advantage to COW.

The closest thing to COW is the concept of smapshot found in the LVM.

Jacques Gelinas <jack_at_solucorp.qc.ca>
vserver: run general purpose virtual servers on one box, full speed!

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