From 1ebfe2d9807a9ba4bfbec3104c6c8df50c89414d Mon Sep 17 00:00:00 2001
From: Kevin Wolf <kwolf@redhat.com>
Date: Fri, 29 May 2015 17:05:12 +0200
Subject: [PATCH 2/8] qcow2: Discard VM state in active L1 after creating
 snapshot

Message-id: <1432919112-18076-3-git-send-email-kwolf@redhat.com>
Patchwork-id: 65149
O-Subject: [RHEL-7.2 qemu-kvm PATCH 2/2] qcow2: Discard VM state in active L1 after creating snapshot
Bugzilla: 1208808
RH-Acked-by: Fam Zheng <famz@redhat.com>
RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
RH-Acked-by: Laszlo Ersek <lersek@redhat.com>

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>
(cherry picked from commit 1ebf561c11302f4fbe4afdd82758fe053cf1d5fc)
Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>

Conflicts:
	block/qcow2.h

Context-only conflict in qcow2.h.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/qcow2-snapshot.c | 7 +++++++
 block/qcow2.c          | 5 -----
 block/qcow2.h          | 5 +++++
 3 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c
index 84481be..6081482 100644
--- a/block/qcow2-snapshot.c
+++ b/block/qcow2-snapshot.c
@@ -407,6 +407,13 @@ 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 babcb4b..6026f8a 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2036,11 +2036,6 @@ 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 9ad8aad..e958ab4 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -417,6 +417,11 @@ 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 uint64_t qcow2_max_refcount_clusters(BDRVQcowState *s)
 {
     return QCOW_MAX_REFTABLE_SIZE >> s->cluster_bits;
-- 
1.8.3.1