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bh=q2c6LbQjApY6VL0+tlMmXw5jrzNaTZpp+9SEMsFYzzQ=; b=mNGrYE9hufr+RbZEORtv6B9z0wuIdxfZOTjhA7ZoPUOrab2wsR4SrTyuj5PKOxrb03 d4hp+WAYLBpQSMfPw59nmWGpepLaEBp3r7h8xXKFCMDXbjiY+nPkY+UEbr4Bi30y6LT6 zxjpNHmtH++4BErELQ7QhDDh532l56K4aOKVdt4ttko1YJdKgfWzPpIxzN5LYYQyNRRc CiyvIbJK7N6HN5rzaZYPTAFNTlfPQl26sw5yscMpYokQSrG97GQ5ZnEQoljEWShbTPgi fcF8h8vDPYj88DLkaVdsvAwxt2gvW386gogDrgNyiNustD4Bpx2TgzFU12lP5YlrYSCO b+KA== X-Gm-Message-State: APjAAAXzCAeDhnsw9lFbh4yUl12/RqoL8yFNIzOLQLD+YFZ9wxsf9WEv kdJNumt/cjxLTKb2tOHMdV6poymlWMrruJV4uKHVYB8zaC/1klFas8dxFp0MAkuQxjpwADvwaek rJPfsVcfSMXXrKQVE3xZ2BYsFTXKaIo8= X-Received: by 2002:a05:6602:24c6:: with SMTP id h6mr2966374ioe.273.1578670212865; Fri, 10 Jan 2020 07:30:12 -0800 (PST) X-Google-Smtp-Source: APXvYqxynYpUDBGmU14s2G9rcYzsvZSfuMk8ZMlCPN6SUeDJQRS5y2Iz0+Ha6ZEMXs3X4ajueWT5hlsCw5SxxGuuSp8= X-Received: by 2002:a05:6602:24c6:: with SMTP id h6mr2966354ioe.273.1578670212589; Fri, 10 Jan 2020 07:30:12 -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> In-Reply-To: <20200110151808.GT3423494@redhat.com> From: Miklos Szeredi Date: Fri, 10 Jan 2020 16:30:01 +0100 Message-ID: Subject: Re: [PATCH 068/104] virtiofsd: passthrough_ll: control readdirplus To: =?UTF-8?Q?Daniel_P=2E_Berrang=C3=A9?= X-MC-Unique: Taz8cpPTOMaphrqZtKvPnA-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: 205.139.110.61 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: Vivek Goyal , "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 4:18 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 Gilbert (g= it) wrote: > > > > > From: Miklos Szeredi > > > > > > > > > > > > > What is readdirplus and what do we need a command line option to > > > > 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 from NF= S > > > where when you iterate over the directory you also pick up stat() dat= a > > > 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 stati= ng > > > each entry (e.g. to figure out if you can access it etc) so rolling i= t > > > into one op avoids the separate stat. The unplus-ness is that it's a= n > > > 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 client > side mount option. This makes sense as only the client really has knowled= ge > 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 adm= in > 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). This needs an extension of the INIT message, so options can be passed to the server. Added this to our TODO list. Thanks, Miklos