From mboxrd@z Thu Jan 1 00:00:00 1970 From: Olaf Hering Subject: Re: new knob to tweak caching mode for backends Date: Wed, 28 May 2014 10:53:40 +0200 Message-ID: <20140528085340.GA29002@aepfle.de> References: <20140526170617.GA29136@aepfle.de> <5384C242.6000102@suse.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Stefano Stabellini Cc: andrew.cooper3@citrix.com, Jim Fehlig , Ian Jackson , Ian Campbell , xen-devel@lists.xen.org List-Id: xen-devel@lists.xenproject.org On Tue, May 27, Stefano Stabellini wrote: > So I think that as long as we keep the default behaviour sane, we can > expose whatever advanced options we like. Its not about changing defaults, but rather provide knobs for the new features. Now that the backend is qdisk (no idea when this became the default) qemu is in charge. qemu offers a cache= option, and backends may honour the value. libvirt seems to have a 1:1 mapping for the qemu backend, I have to check if libvirt does any processing or if it passes cache= as is. Now from libirt->libxl->qemu there is no way to pass cache= because libxl got no cache= knob while qdisk became the default. Again, I have to check what the libxl driver in libvirt does if cache= specified, if its ignored or handled as error. So with this in mind, will a patch be accepted to add some sort of "char *cache" member to struct libxl_disk_device? Should it accept all values libvirt and qemu knows about, or just the few values qdisk can actually handle (nocache|write-back|directsync)? In addition to that: the direct-io-safe knob does not only enable AIO it also changes from write-back to directsync. Qemu has a knob aio=native and cache= for directsync. Looks like direct-io-safe merges both. Is that intentional? If libxl gets a cache= option, should it be possible to specify "direct-io-safe,cache=write-back"? Olaf