From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41106) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eHs2A-0007mR-I4 for qemu-devel@nongnu.org; Thu, 23 Nov 2017 08:59:19 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1eHs29-0001aR-9G for qemu-devel@nongnu.org; Thu, 23 Nov 2017 08:59:18 -0500 Received: from mx1.redhat.com ([209.132.183.28]:50986) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1eHs28-0001Z9-Tn for qemu-devel@nongnu.org; Thu, 23 Nov 2017 08:59:17 -0500 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 06082FC7BA for ; Thu, 23 Nov 2017 13:59:16 +0000 (UTC) From: Fam Zheng Date: Thu, 23 Nov 2017 21:59:05 +0800 Message-Id: <20171123135906.12852-2-famz@redhat.com> In-Reply-To: <20171123135906.12852-1-famz@redhat.com> References: <20171123135906.12852-1-famz@redhat.com> Subject: [Qemu-devel] [PATCH 1/2] docs: Add image locking subsection List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: kwolf@redhat.com, famz@redhat.com, mreitz@redhat.com This documents the image locking feature and explains when and how related options can be used. Signed-off-by: Fam Zheng --- docs/qemu-block-drivers.texi | 36 ++++++++++++++++++++++++++++++++++++ qemu-doc.texi | 1 + 2 files changed, 37 insertions(+) diff --git a/docs/qemu-block-drivers.texi b/docs/qemu-block-drivers.texi index 1cb1e55686..fa2e90d15f 100644 --- a/docs/qemu-block-drivers.texi +++ b/docs/qemu-block-drivers.texi @@ -785,6 +785,42 @@ warning: ssh server @code{ssh.example.com:22} does not support fsync With sufficiently new versions of libssh2 and OpenSSH, @code{fsync} is supported. +@node disk_image_locking +@subsection Disk image file locking + +By default, QEMU tries to protect image files from unexpected concurrent +access, as long as it's supported by the block protocol driver and host +operating system. If multiple QEMU processes (including QEMU emulators and +utilities) try to open the same image with conflicting accessing modes, all but +the first one will get an error. + +This feature is currently supported by the file protocol on Linux with Open +File Descriptor (OFD) locking API, and can be configured to fall back to POSIX +locking if the POSIX host doesn't support Linux OFD locking. + +To explicitly enable image locking, specify "locking=on" in the file protocol +driver options. If OFD locking is not possible, a warning will be printed and +the POSIX locking API will be used. In this case there is risk that the lock +will get silently lost when doing hot plugging and block jobs, due to the +shortcomings of the POSIX locking API. + +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. + +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 +under a format driver. For example: + +@code{-blockdev driver=qcow2,file.filename=/path/to/image,file.locking=off,file.driver=file} + +To check if image locking is active, check the output of "lslocks" command on +host and see if there are locks held by QEMU process on the image file. More +than one bytes could be locked by a QEMU instance, each byte of which reflects +a perticular permission that are acquired or protected by the running block +driver. + @c man end @ignore diff --git a/qemu-doc.texi b/qemu-doc.texi index 617254917d..db2351c746 100644 --- a/qemu-doc.texi +++ b/qemu-doc.texi @@ -405,6 +405,7 @@ encrypted disk images. * disk_images_iscsi:: iSCSI LUNs * disk_images_gluster:: GlusterFS disk images * disk_images_ssh:: Secure Shell (ssh) disk images +* disk_image_locking:: Disk image file locking @end menu @node disk_images_quickstart -- 2.14.3