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Re: [Qemu-devel] [PATCH for-2.7 v2 07/17] rbd: Implement image locking

From: Jason Dillaman
Subject: Re: [Qemu-devel] [PATCH for-2.7 v2 07/17] rbd: Implement image locking
Date: Fri, 22 Apr 2016 21:57:54 -0400

Since this cannot automatically recover from a crashed QEMU client with an RBD image, perhaps this RBD locking should not default to enabled. Additionally, this will conflict with the "exclusive-lock" feature available since the Ceph Hammer-release since both utilize the same locking construct.  

As a quick background, the optional exclusive-lock feature can be enabled/disabled on image and safely/automatically handles the case of recovery from a crashed client.  Under normal conditions, the RBD exclusive-lock feature automatically acquires the lock upon the first attempt to write to the image and transparently transitions ownership of the lock between two or more clients -- used for QEMU live-migration.

I'd be happy to add a new librbd API method to explicitly expose acquiring and releasing the RBD exclusive lock since it certainly solves a couple compromises in our current QEMU integration.

On Thu, Apr 14, 2016 at 11:27 PM, Fam Zheng <address@hidden> wrote:
librbd has the locking API that can be used to implement .bdrv_lockf.

Signed-off-by: Fam Zheng <address@hidden>
 block/rbd.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/block/rbd.c b/block/rbd.c
index 5bc5b32..a495083 100644
--- a/block/rbd.c
+++ b/block/rbd.c
@@ -810,6 +810,30 @@ static int qemu_rbd_truncate(BlockDriverState *bs, int64_t offset)
     return 0;

+static int qemu_rbd_lockf(BlockDriverState *bs, BdrvLockfCmd cmd)
+    int ret;
+    BDRVRBDState *s = bs->opaque;
+    /* XXX: RBD locks are not released automatically when program exits, which
+     * means if QEMU dies it cannot open the image next time until manually
+     * unlocked. */
+    switch (cmd) {
+        ret = rbd_lock_exclusive(s->image, NULL);
+        break;
+        ret = rbd_lock_shared(s->image, NULL, NULL);
+        break;
+        ret = rbd_unlock(s->image, NULL);
+        break;
+    default:
+        abort();
+    }
+    return ret;
 static int qemu_rbd_snap_create(BlockDriverState *bs,
                                 QEMUSnapshotInfo *sn_info)
@@ -998,6 +1022,7 @@ static BlockDriver bdrv_rbd = {
     .bdrv_aio_discard       = qemu_rbd_aio_discard,

+    .bdrv_lockf             = qemu_rbd_lockf,
     .bdrv_snapshot_create   = qemu_rbd_snap_create,
     .bdrv_snapshot_delete   = qemu_rbd_snap_remove,
     .bdrv_snapshot_list     = qemu_rbd_snap_list,



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