Bernd Schubert <bs_lists@xxxxxxxxxxxxxxxxx> writes:
> On Saturday 28 February 2009, Goswin von Brederlow wrote:
>> Miklos Szeredi <miklos@xxxxxxxxxx> 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
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