From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:57534) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ek1Fp-00058K-37 for qemu-devel@nongnu.org; Fri, 09 Feb 2018 00:29:48 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ek1Fo-00044x-56 for qemu-devel@nongnu.org; Fri, 09 Feb 2018 00:29:45 -0500 From: Fam Zheng Date: Fri, 9 Feb 2018 13:29:15 +0800 Message-Id: <20180209052915.12855-4-famz@redhat.com> In-Reply-To: <20180209052915.12855-1-famz@redhat.com> References: <20180209052915.12855-1-famz@redhat.com> Subject: [Qemu-devel] [PATCH v7 3/3] docs: Document share-rw property more thoroughly List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: stefanha@redhat.com, qemu-block@nongnu.org, kchamart@redhat.com, eblake@redhat.com, Kevin Wolf , Max Reitz Suggested-by: Stefan Hajnoczi Signed-off-by: Fam Zheng --- docs/qemu-block-drivers.texi | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/docs/qemu-block-drivers.texi b/docs/qemu-block-drivers.texi index cd74767ed3..f1793692bb 100644 --- a/docs/qemu-block-drivers.texi +++ b/docs/qemu-block-drivers.texi @@ -845,6 +845,16 @@ QEMU transparently handles lock handover during shared storage migration. For shared virtual disk images between multiple VMs, the "share-rw" device option should be used. +By default, the guest has exclusive write access to its disk image. If the +guest can safely share the disk image with other writers the @code{-device +...,share-rw=on} parameter can be used. This is only safe if the guest is +running software, such as a cluster file system, that coordinates disk accesses +to avoid corruption. + +Note that share-rw=on only declares the guest's ability to share the disk. +Some QEMU features, such as image file formats, require exclusive write access +to the disk image and this is unaffected by the share-rw=on option. + Alternatively, locking can be fully disabled by "locking=off" block device option. In the command line, the option is usually in the form of "file.locking=off" as the protocol driver is normally placed as a "file" child -- 2.14.3