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>

diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c
index e7e601301a9467b6c5ebad7cec0f3431bbb75999..ffead08ca2cdd9a292cb482efe70a54119757a50 100644 (file)
--- 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 147822e8fd9ec045c0fc2713aa493a49d049d542..c9e266e22e6ee32f8dc8a64bc8fa1486b339067b 100644 (file)
--- 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 9c33b984578a972d131230b911a4e823b0d83fba..49eed828c567aa993dcbcb27ea0ab981064adf05 100644 (file)
--- 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) {