Re: [fuse-devel] delta filesystem prototype

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

Bernd Schubert <bs_lists@aakef.fastmail.fm> writes:

> On Saturday 28 February 2009, Goswin von Brederlow wrote:
>> Miklos Szeredi <miklos@szeredi.hu> writes:
>> > Here is my first try at a "delta" filesystem.  It takes two
>> > directories, one of which is a read-only base, and the other is where
>> > the differences are stored.  It stores data, metadata and directory
>> > modifications without copying up whole files from the read-only
>> > branch.
>> >
>> > The layout of the delta store may look similar to the writable branch
>> > of a union fs, but this is basically just coincidence (it was easier
>> > to start out this way).
>> >
>> > Currently it's implemented with fuse and it's not optimized at all, so
>> > performance may suck in some cases.  But I think this is a useful
>> > concept and a better model, than trying to fit writable branches into
>> > a union filesystem.
>> >
>> > Comments, bug reports are welcome.
>> >
>> > Thanks,
>> > Miklos
>>
>> Wouldn't it make more sense to start with unionfs-fuse and add a delta
>> feature to it? unionfs-fuse already has all you need except that it
>> will copy the whole file (if on a read-only branch) on write.
>
> Well yes, but it would need to be configurable by the user. IMMHO, the 'delta' 
> ansatz has a big problem - what happens if the admin decides to modify the 
> underlying ro-branch, which is a distribution chroot seen by all clients as 
> their '/'? Any time files may be modified or even deleted on this branch when 
> the admin does an update.
> This is also the exactly the problem why none of the kernel unionfs 
> implementation fits my needs and why I started to work on unionfs-fuse. Only, 
> the delta approach makes it even worse ;)
>
>
> Cheers,
> Bernd

Obviously with delta storage the underlying branches really MUST be
read-only. As for configuration I see no problem in clasifying a
branch as RO, RW or D. If the user wants a modifiable RO branch then
he can not have any D branch. Easy enough.

MfG
        Goswin