Re: [fuse-devel] delta filesystem prototype

[Goswin v. B. <gos...@we...>]

hoo...@ya... writes:

> Goswin von Brederlow:
>> Why? When a page is accessed the filesystem gets a read request and
>> reads it either from the RO branch or the delta branch. Why should it
>> need to copy up the full file?
>
> For example,
> - you have two mmap for a single file

On open you have to create an internal FD structure that keeps track
of the delta informtaions and the read-only and delta filedescriptors
(one or both can be -1). If the same file is opened again you have to
use the same internal FD structure so both share the same delta
informations.

> - the first mapping is done, it may map the file on the lower rdonly
>   layer
> - the other mapping modifies the contents
> - can the first mapping see the new content?

Both mmaps would be to the internal FD and thus share any change made
by one of them.

> Of course, it may depend on the implementation.

Sure. If you don't create an internal FD structure and share it
between open calls things will break. Not just mmap.

MfG
        Goswin