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

[Pavel Emelyanov]

>>> 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.

> Regards,
> 
> Tvrtko

Thanks,
Pavel

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