Re: [RFC] VFS: FS CoW using redirection

[Al Boldi <a1426z@gawab.com>]

Eric Van Hensbergen wrote:
> On 7/8/06, Al Boldi <a1426z@gawab.com> wrote:
> > Copy on Write is a neat way to quickly achieve a semi-clustered system,
> > by mounting any shared FS read-only and redirecting writes to some
> > perClient FS.
> >
> > Would this redirection be easy to implement into the VFS?
>
> There are a variety of solutions that have been proposed or are
> available to do this sort of thing.  You may want to start by looking
> at unionfs and mapfs.  There are also folks looking at doing this from
> the block layer (look at the dm-userspace + cowd as well as evms and
> lvm snapshots).

The idea here, is to do things completely transparent while being as 
unintrusive as possible, by relying on the current FS infrastructure, w/o 
introducing another VFS.

Consider something simple like this:

VFS - anyFS1 (r/w) used normally, unless ENotFound, then redirect read to
    \              anyFS2, or CoW from anyFS2 to anyFS1.
      anyFS2 (r/o) used normally.

i.e: Does the VFS provide hooks for perMount Handlers?


Thanks!

--
Al