From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:53362) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YdFLp-00009E-Vm for qemu-devel@nongnu.org; Wed, 01 Apr 2015 05:54:24 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YdFLp-0004hy-2M for qemu-devel@nongnu.org; Wed, 01 Apr 2015 05:54:21 -0400 Message-ID: <551BC040.9000806@msgid.tls.msk.ru> Date: Wed, 01 Apr 2015 12:54:08 +0300 From: Michael Tokarev MIME-Version: 1.0 References: <55151DB7.3090200@msgid.tls.msk.ru> <55156DE0.4050209@redhat.com> <551578F4.9010400@msgid.tls.msk.ru> <55158F8B.9090607@redhat.com> <20150330153634.GC4305@noname.redhat.com> <551BB9DA.7030309@msgid.tls.msk.ru> In-Reply-To: <551BB9DA.7030309@msgid.tls.msk.ru> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] block-commit & dropping privs List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf , Eric Blake Cc: qemu-devel , qemu-block@nongnu.org 01.04.2015 12:26, Michael Tokarev =D0=BF=D0=B8=D1=88=D0=B5=D1=82: > 30.03.2015 18:36, Kevin Wolf wrote: >> Am 27.03.2015 um 18:12 hat Eric Blake geschrieben: >>> On 03/27/2015 09:36 AM, Michael Tokarev wrote: >>>> Wonder how to specify cache mode, or should I open these with proper >>>> O_DIRECT/O_SYNC/whatever? It looks like it's possible to change O_D= IRECT >>>> at runtime but not O_SYNC. >>>> >>>> And the more interesting question is how to do that from shell. >>> >>> Redirections only get you so far in shell; you may need a wrapper C >>> program go get O_DIRECT and/or O_SYNC pre-set. Then again, if you us= e >>> QMP and pass over the Unix socket, you need a C program anyways. >> >> O_DIRECT can be set with fcntl(), so qemu takes care of that. O_SYNC >> is completely unused on Linux these days, so that shouldn't be a probl= em Do you mean it is unused in qemu or in kernel? :) >> either. (Other platforms use it as a misguided attempt of approximatin= g >> O_DIRECT. We should really error out instead.) >> >> So if I'm not mistaken, just having one read-only and one read-write f= d >> should be enough for any configuration in practice. Yes, O_DIRECT appears to work with fdsets. > Probably yes. Except the thing doesn't actually work. ;) >=20 > When flushing changes to the base image, that base image needs to be > reopened. So I should convince qemu that the base image of this qcow > file is /dev/fdset/foo, not the one recorded in the header. qemu-system-x86_64: -drive file=3Dw7x64.qcow2,backing_file=3D/dev/fdset/2= ,if=3Dnone,id=3Ddr,cache=3Dnone: \ could not open disk image w7x64.qcow2: Block format 'qcow2' used by devi= ce 'dr' \ doesn't support the option 'backing_file' hmm?.. > Is it possible? Thanks, /mjt >=20