Re: [fuse-devel] delta filesystem prototype

[Goswin von Brederlow <goswin-v-b@web.de>]

Miklos Szeredi <miklos@szeredi.hu> writes:

> On Wed, 04 Mar 2009, Goswin von Brederlow wrote:
>> Bernd and I ment the following scenario:
>> 
>> /dev/sda1 /union/read-only
>> tmpfs     /union/read-write
>> 
>> with a delta-fs merging the two. Then running "echo foo >
>> /union/read-only/path/file" could be desasterous to your data.
>
> Well, if the writable branch is really meant to be a clone of the
> underlying fs, then yes.  But writable unions are _not_ clones either,
> very far from that.

The problem is that /delta-fs/path/file would suddenly be a composite
of the new file /union/read-only/path/file and any stored delta
information in /union/read-write/path/file of the old file.

In unionfs-fuse files are currently always completly copy-up-ed when
modified. There a change of /union/read-only/path/file will give the
new file if it wasn't modified or the old modified file if it was. But
never mix the two.

>> The underlying branch must not change on its own. The delta-fs could
>> change it on command but that wasn't what we where thinking of.
>
> Delta-fs could deal with changes in the underlying filesystem, in some
> cases much better than unionfs (for example if the file was renamed in
> the delta-fs, and it was later modified in the underlying fs).
>
> Thanks,
> Miklos

Ugh, no. The delta-fs has no way of knowing when a file in a branch
will be changed outside of delta-fs. And it would have to know that
before the change so it can copy-up relevant data before they change.

MfG
        Goswin