Message-ID: <50A378C0.70406@parallels.com>
Date: Wed, 14 Nov 2012 14:56:00 +0400
From: Pavel Emelyanov <xemul@parallels.com>
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To: Tvrtko Ursulin <tvrtko.ursulin@onelan.co.uk>
CC: Cyrill Gorcunov <gorcunov@openvz.org>,
        David Rientjes <rientjes@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Al Viro <viro@zeniv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        James Bottomley <jbottomley@parallels.com>,
        Matthew Helsley <matt.helsley@gmail.com>,
        aneesh.kumar@linux.vnet.ibm.com, bfields@fieldses.org
Subject: Re: [patch 3/7] fs, notify: Add file handle entry into inotify_inode_mark
References: <20121112101440.665694060@openvz.org> <2092535.8S9EcgmZCZ@deuteros> <50A37692.1010500@parallels.com> <1491483.8kFV7tRC1p@deuteros>
In-Reply-To: <1491483.8kFV7tRC1p@deuteros>
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>>> 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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