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bh=z9CvWAvOV23LADIsDc0mAhU0nx4Z0CbtTvdf6J+kNis=; b=b1KiSOptQMnrfNg+fVdgcpfayuFXiDg7hzCwDr5hJlPQQGBtilDaTQYeObOPbPMzno vL/OZRVxJKjHZgmtw2GW2IZva67hafNkO9TgHa4Mu92Hop62lw84Gsj5Px36v9wk06zW lyjcQmt9uZLSW+GNxcRN/qMS0BcJU7B7HfY71X9hoX8QzRzk143ixPMOhnKBN7nLMvvD O4pNR2Iv3qqxU+BUyTkNdne77l46RQf4ezThhOZe2DqJzsGsx6yso1Y76Qgjm2O5FBEw DliHdGkUO9WwYDGLDflUpadfuWHWmR2Q1SrRrGGAonBH46XMDITH/QkVaK+soXUYSzaE FoRQ== X-Gm-Message-State: APjAAAVDi2iYWcpqPTsNQ5Sv0ZkCLuDv30B+t4EMPKPgWxux3VwAHAaL 2OwHaF9BsT2ayqdAWt2PjzBvssqX4bA3A1sA2YXtlgPanmSGAz6cqtjn1NZlsnQP0ep2P/Aymvc mnIVP12YTIpjr61Eiww7kna0WZGrG2WE= X-Received: by 2002:a92:9c04:: with SMTP id h4mr3375324ili.6.1578672044878; Fri, 10 Jan 2020 08:00:44 -0800 (PST) X-Google-Smtp-Source: APXvYqyvjZ77PT1S9mGgx9Aqra7xS1pvcTPOmC9SmqrOlLCH388nmlWiWthXpPmDx5F81NWik3NBGIWHBGihaZxyZUQ= X-Received: by 2002:a92:9c04:: with SMTP id h4mr3375218ili.6.1578672043861; Fri, 10 Jan 2020 08:00:43 -0800 (PST) MIME-Version: 1.0 References: <20191212163904.159893-1-dgilbert@redhat.com> <20191212163904.159893-69-dgilbert@redhat.com> <20200107112316.GL3368802@redhat.com> <20200110150447.GH3901@work-vm> <20200110151808.GT3423494@redhat.com> <20200110154037.GC28043@redhat.com> In-Reply-To: <20200110154037.GC28043@redhat.com> From: Miklos Szeredi Date: Fri, 10 Jan 2020 17:00:32 +0100 Message-ID: Subject: Re: [PATCH 068/104] virtiofsd: passthrough_ll: control readdirplus To: Vivek Goyal X-MC-Unique: jOnPkHwlNqy4XE1CbICFLA-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.81 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: =?UTF-8?Q?Daniel_P=2E_Berrang=C3=A9?= , "Dr. David Alan Gilbert" , Stefan Hajnoczi , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Jan 10, 2020 at 4:40 PM Vivek Goyal wrote: > > On Fri, Jan 10, 2020 at 04:30:01PM +0100, Miklos Szeredi wrote: > > On Fri, Jan 10, 2020 at 4:18 PM Daniel P. Berrang=C3=A9 wrote: > > > > > > On Fri, Jan 10, 2020 at 04:13:08PM +0100, Miklos Szeredi wrote: > > > > On Fri, Jan 10, 2020 at 4:04 PM Dr. David Alan Gilbert > > > > wrote: > > > > > > > > > > * Daniel P. Berrang=C3=A9 (berrange@redhat.com) wrote: > > > > > > On Thu, Dec 12, 2019 at 04:38:28PM +0000, Dr. David Alan Gilber= t (git) wrote: > > > > > > > From: Miklos Szeredi > > > > > > > > > > > > > > > > > > > What is readdirplus and what do we need a command line option t= o > > > > > > control it ? What's the user benefit of changing the setting ? > > > > > > > > > > cc'ing Miklos who understands this better than me. > > > > > > > > > > My understanding is that readdirplus is a heuristic inherited fro= m NFS > > > > > where when you iterate over the directory you also pick up stat()= data > > > > > for each entry in the directory. You then cache that stat data > > > > > somewhere. > > > > > The Plus-ness is that a lot of directory operations involve you s= tating > > > > > each entry (e.g. to figure out if you can access it etc) so rolli= ng it > > > > > into one op avoids the separate stat. The unplus-ness is that it= 's an > > > > > overhead and I think changes some of the caching behaviour. > > > > > > > > Yeah, so either may give better performance and it's hard to pick a > > > > clear winner. NFS also has an option to control this. > > > > > > IIUC from the man page, the NFS option for controlling this is a clie= nt > > > side mount option. This makes sense as only the client really has kno= wledge > > > of whether its workload will benefit. > > > > > > With this in mind, should the readdirplus control for virtio-fs also = be a > > > guest mount option instead of a host virtiofsd CLI option ? The guest= admin > > > seems best placed to know whether their workload will benefit or not. > > > > Definitely. In fact other options, e.g. ones that control caching, > > should probably also be client side (cache=3DXXX, writeback, > > timeout=3DXXX, etc). > > I am not sure about cache options. So if we want to share a directory > between multiple guests with stronger coherency (cache=3Dnone), then admi= n > should decide that cache=3Dalways/auto is not supported on this export. > > Also, how will one client know whether there are other clients same > directory with strong coherency requirements and it should use cache=3Dno= ne > instead of cache=3Dalways/auto. > > Having said that, it also makes sense that client knows its workoad > and can decide if cache=3Dauto works best for it and use that instead. > > May be we need both client and server side options. Client will request > certain cache=3Dxxx options and server can deny these if admin decides > not to enable that option for that particular mount. > > For example, if admin decides that we can only support cache=3Dnone on > this particular dir due to other guest sharing it, then daemon should > be able to deny cache=3Dauto/always requests from client. Makes sense. The server dictates policy, the client just passes the options onto the server. Thanks, Miklos