From mboxrd@z Thu Jan 1 00:00:00 1970 From: Pavel Emelyanov Subject: Re: [patch 3/7] fs, notify: Add file handle entry into inotify_inode_mark Date: Wed, 14 Nov 2012 14:56:00 +0400 Message-ID: <50A378C0.70406@parallels.com> References: <20121112101440.665694060@openvz.org> <2092535.8S9EcgmZCZ@deuteros> <50A37692.1010500@parallels.com> <1491483.8kFV7tRC1p@deuteros> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Cyrill Gorcunov , David Rientjes , Andrew Morton , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Al Viro , Alexey Dobriyan , James Bottomley , Matthew Helsley , aneesh.kumar@linux.vnet.ibm.com, bfields@fieldses.org To: Tvrtko Ursulin Return-path: In-Reply-To: <1491483.8kFV7tRC1p@deuteros> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org >>> 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