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authorKevin Wolf <kwolf@redhat.com>2013-09-06 12:20:08 +0200
committerKevin Wolf <kwolf@redhat.com>2013-09-12 10:12:46 +0200
commit1ebf561c11302f4fbe4afdd82758fe053cf1d5fc (patch)
tree97ddd797d3feefe5751a858b3b2df42efbd3dea1
parent670df5e3b4b5ef830a7c3c970170dbfa11cbb8d2 (diff)
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qcow2: Discard VM state in active L1 after creating snapshot
During savevm, the VM state is written to the active L1 of the image and then a snapshot is taken. After that, the VM state isn't needed any more in the active L1 and should be discarded. This is implemented by this patch. The impact of not discarding the VM state is that a snapshot can never become smaller than any previous snapshot (because it would be padded with old VM state), and more importantly that future savevm operations cause unnecessary COWs (with associated flushes), which makes subsequent snapshots much slower. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Reviewed-by: Max Reitz <mreitz@redhat.com>
-rw-r--r--block/qcow2-snapshot.c7
-rw-r--r--block/qcow2.c5
-rw-r--r--block/qcow2.h5
3 files changed, 12 insertions, 5 deletions
diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c
index e7e6013..ffead08 100644
--- a/block/qcow2-snapshot.c
+++ b/block/qcow2-snapshot.c
@@ -416,6 +416,13 @@ int qcow2_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info)
g_free(old_snapshot_list);
+ /* The VM state isn't needed any more in the active L1 table; in fact, it
+ * hurts by causing expensive COW for the next snapshot. */
+ qcow2_discard_clusters(bs, qcow2_vm_state_offset(s),
+ align_offset(sn->vm_state_size, s->cluster_size)
+ >> BDRV_SECTOR_BITS,
+ QCOW2_DISCARD_NEVER);
+
#ifdef DEBUG_ALLOC
{
BdrvCheckResult result = {0};
diff --git a/block/qcow2.c b/block/qcow2.c
index 147822e..c9e266e 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1757,11 +1757,6 @@ static coroutine_fn int qcow2_co_flush_to_os(BlockDriverState *bs)
return 0;
}
-static int64_t qcow2_vm_state_offset(BDRVQcowState *s)
-{
- return (int64_t)s->l1_vm_state_index << (s->cluster_bits + s->l2_bits);
-}
-
static int qcow2_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
{
BDRVQcowState *s = bs->opaque;
diff --git a/block/qcow2.h b/block/qcow2.h
index 9c33b98..49eed82 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -361,6 +361,11 @@ static inline int64_t align_offset(int64_t offset, int n)
return offset;
}
+static inline int64_t qcow2_vm_state_offset(BDRVQcowState *s)
+{
+ return (int64_t)s->l1_vm_state_index << (s->cluster_bits + s->l2_bits);
+}
+
static inline int qcow2_get_cluster_type(uint64_t l2_entry)
{
if (l2_entry & QCOW_OFLAG_COMPRESSED) {