Re: [RFC] VFS: FS CoW using redirection

[Al Boldi <a1426z@gawab.com>]

Al Boldi wrote:
> 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?

Silence is probably an indication that the VFS does not provide hooks.

The question then:

1. Would it be difficult to introduce perMount Handler hooks into the VFS?

2. Would it be easier to replace the current VFS with one that offers a more 
flexible/open configuration?

3. Would these changes be welcome, or is the VFS considered to be a forbidden 
zone?


Thanks!

--
Al