Re: [vserver] COW links, beyond vservers?

From: Daniel Hokka Zakrisson <>
Date: Sun 20 Apr 2008 - 15:26:35 BST
Message-ID: <57801.>

Martin Fick wrote:
> Are there any docs explaining how COW links works.
> I'm sorry if there are, I have looked and can't seem
> to find anything more than a mention of them and
> vunify/vhashify. Are there any tools to create and
> manipulate COW links manually without using the
> vunify/hashify programs?

setattr --iunlink. Note that it requires that you have
CAP_LINUX_IMMUTABLE, since that's one of the bits it uses.

> Essentially, I would like to create and use COW links
> outside of vservers (they are a great idea). I
> realize that I probably need a vserver kernel for this
> (I use vservers also, so no problem). But this seems
> like a very useful feature even without vservers.
> Are there any thoughts about trying to integrate this
> feature as a separate feature into the mainline linux
> kernel? It hardly seems like something that only
> vserver users would want. Is it a huge patch to
> enable COW links or would it be something minor that
> might be accepted easily?

You need to touch every single filesystem, in addition to adding the hooks
in open_namei and similar. The fact that you need to be root to create
them makes it unlikely that it would be accepted as generally useful...

> Thanks,
> -Martin

Daniel Hokka Zakrisson
Received on Sun Apr 20 15:26:46 2008
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