Re: [patch 3/7] fs, notify: Add file handle entry into inotify_inode_mark

[Cyrill Gorcunov <gorcunov@openvz.org>]

On Wed, Nov 14, 2012 at 02:56:00PM +0400, Pavel Emelyanov wrote:
> >>> How much space does a typical file system need to encode a handle? Am I
> >>> right that for must it is just a few bytes? (I just glanced at the code
> >>> so I might be wrong.) In which case, could the handle buffer be allocated
> >>> dynamically depending on the underlying filesystem? Perhaps adding a
> >>> facility to query a filesystem about its maximum handle buffer needs? Do
> >>> you think the saving would justify this extra work?
> >>
> >> Well, the MAX_HANDLE_SZ is taken from NFSv4 and is 128 bytes which is quite
> >> big for inotify extension indeed. The good news is that this amount of bytes
> >> seem to be required for the most descriptive fhandle -- with info about
> >> parent, etc. We don't need such, we can live with shorter handle, people
> >> said that 40 bytes was enough for that.
> >>
> >> However, your idea about determining the handle size dynamically seems
> >> promising. As far as I can see from the code we can call for encode_fh with
> >> size equals zero and filesystem would report back the amount of bytes it
> >> requires for a handle.
> >>
> >> We can try going this route, what do you think?
> > 
> > Sounds much better since that would only add one pointer to the watch 
> > structure in the normal case.
> > 
> > Also at checkpoint time it will use only a few bytes (compared to 64) for the 
> > encode buffer for most filesystems. This part is probably not that important 
> > but still a win.
> 
> No, the thing is -- we need to know the handle _before_ we start checkpoint.
> More exactly -- at the time the inotify_add_watch is called. So the memory save
> would be not that big.

for the worst case (NFSv4) it'll be 128 bytes + 8|4 byte pointer, but for
more common cases such as extXfs it'll be shrinked down to 8 byte handle +
8|4 byte pointer, which is a pretty good i think.