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From 16d78f7cd9e1455ebb0599706ba5badfa3ee4fdc Mon Sep 17 00:00:00 2001
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From: Kevin Wolf <kwolf@redhat.com>
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Date: Fri, 6 Sep 2013 12:32:26 +0200
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Subject: [PATCH] qcow2: Discard VM state in active L1 after creating snapshot
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During savevm, the VM state is written to the active L1 of the image and
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then a snapshot is taken. After that, the VM state isn't needed any more
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in the active L1 and should be discarded. This is implemented by this
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patch.
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The impact of not discarding the VM state is that a snapshot can never
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become smaller than any previous snapshot (because it would be padded
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with old VM state), and more importantly that future savevm operations
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cause unnecessary COWs (with associated flushes), which makes subsequent
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snapshots much slower.
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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---
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block/qcow2-snapshot.c | 7 +++++++
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block/qcow2.c | 5 -----
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block/qcow2.h | 5 +++++
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3 files changed, 12 insertions(+), 5 deletions(-)
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diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c
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index 0caac90..ae33b45 100644
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--- a/block/qcow2-snapshot.c
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+++ b/block/qcow2-snapshot.c
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@@ -401,6 +401,13 @@ int qcow2_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info)
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g_free(old_snapshot_list);
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+ /* The VM state isn't needed any more in the active L1 table; in fact, it
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+ * hurts by causing expensive COW for the next snapshot. */
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+ qcow2_discard_clusters(bs, qcow2_vm_state_offset(s),
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+ align_offset(sn->vm_state_size, s->cluster_size)
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+ >> BDRV_SECTOR_BITS,
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+ QCOW2_DISCARD_NEVER);
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+
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#ifdef DEBUG_ALLOC
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{
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BdrvCheckResult result = {0};
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diff --git a/block/qcow2.c b/block/qcow2.c
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index 16e45a0..f63c2cb 100644
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--- a/block/qcow2.c
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+++ b/block/qcow2.c
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@@ -1666,11 +1666,6 @@ static coroutine_fn int qcow2_co_flush_to_os(BlockDriverState *bs)
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return 0;
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}
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-static int64_t qcow2_vm_state_offset(BDRVQcowState *s)
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-{
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- return (int64_t)s->l1_vm_state_index << (s->cluster_bits + s->l2_bits);
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-}
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-
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static int qcow2_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
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{
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BDRVQcowState *s = bs->opaque;
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diff --git a/block/qcow2.h b/block/qcow2.h
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index 52cf193..da61d18 100644
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--- a/block/qcow2.h
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+++ b/block/qcow2.h
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@@ -324,6 +324,11 @@ static inline int64_t align_offset(int64_t offset, int n)
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return offset;
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}
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+static inline int64_t qcow2_vm_state_offset(BDRVQcowState *s)
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+{
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+ return (int64_t)s->l1_vm_state_index << (s->cluster_bits + s->l2_bits);
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+}
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+
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static inline int qcow2_get_cluster_type(uint64_t l2_entry)
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{
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if (l2_entry & QCOW_OFLAG_COMPRESSED) {
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