From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41168) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1b0P83-0000hv-F4 for qemu-devel@nongnu.org; Wed, 11 May 2016 04:04:24 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1b0P81-0000Rd-8T for qemu-devel@nongnu.org; Wed, 11 May 2016 04:04:22 -0400 From: Markus Armbruster References: <20160510091422.GG4921@noname.str.redhat.com> <20160510092338.GG13377@redhat.com> <20160510093514.GH4921@noname.str.redhat.com> <20160510094310.GH13377@redhat.com> <20160510100706.GJ4921@noname.str.redhat.com> <20160510101612.GY1683@redhat.com> <20160510110849.GK4921@noname.str.redhat.com> <20160510114615.GZ1683@redhat.com> <20160510120102.GM4921@noname.str.redhat.com> <20160510121130.GA1683@redhat.com> <20160510122209.GJ13377@redhat.com> Date: Wed, 11 May 2016 10:04:12 +0200 In-Reply-To: <20160510122209.GJ13377@redhat.com> (Daniel P. Berrange's message of "Tue, 10 May 2016 13:22:09 +0100") Message-ID: <87mvnxhvgj.fsf@dusky.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain Subject: Re: [Qemu-devel] [PATCH v4 00/27] block: Lock images when opening List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Daniel P. Berrange" Cc: "Richard W.M. Jones" , Kevin Wolf , Fam Zheng , qemu-block@nongnu.org, Jeff Cody , qemu-devel@nongnu.org, stefanha@redhat.com, den@openvz.org, pbonzini@redhat.com, Max Reitz , John Snow "Daniel P. Berrange" writes: > On Tue, May 10, 2016 at 01:11:30PM +0100, Richard W.M. Jones wrote: >> At no point did I say that it was safe to use libguestfs on live VMs >> or that you would always get consistent data out. >> >> But the fact that it can fail is understood, the chance of failure is >> really tiny (it has literally only happened twice that I've read >> corrupted data, in years of daily use), and the operation is very >> useful. >> >> So I think this patch series should either not lock r/o VMs, or should >> add a nolock flag to override the locking (which libguestfs will >> always use). > > If QEMU locks r/o disks, then libvirt would likely end up setting the > "nolock" flag unconditionally too, in order to avoid breaking libguestfs > and other application usage of libvirt. Could a QEMU + libvirt together provide both safe and unsafe read-only access? Safe means you get consistent data. Unsafe means you're taking your chances. Libguestfs could then use unsafe if the user asks for it. Or even by default; that's really libguesfs's business. Backward compatibility may complicate things, but getting into a reasonable state is sometimes worth a lengthy and somewhat messy transition.