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[Qemu-devel] [PATCH v2] qcow2: discard bitmap when removed

From: Andrey Shinkevich
Subject: [Qemu-devel] [PATCH v2] qcow2: discard bitmap when removed
Date: Thu, 28 Feb 2019 12:26:59 +0300

When a bitmap is removed, we can clean some space on the disk. The size
of a cluster may be larger, so is the size of the bitmap that includes
many clusters. Some bitmaps can be as large as tens of megabytes.
The flag QCOW2_DISCARD_ALWAYS allows a call to the raw_co_pdiscard()
that does the actual cleaning of the image on disk, while with the flag
QCOW2_DISCARD_OTHER, a reference count of the cluster is updated only.

Signed-off-by: Andrey Shinkevich <address@hidden>
v1: Discard old bitmap directories in QCOW2 image

In the first version of the patch, any call to qcow2_free_clusters() in
block/qcow2-bitmap.c was with the flag QCOW2_DISCARD_ALWAYS, even in the
cases when the cleaned space is insignificant, particularly, in case of
bitmap directories.
Discussed in the email thread with the message ID

 block/qcow2-bitmap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/qcow2-bitmap.c b/block/qcow2-bitmap.c
index 3ee524d..162744e 100644
--- a/block/qcow2-bitmap.c
+++ b/block/qcow2-bitmap.c
@@ -202,7 +202,7 @@ static void clear_bitmap_table(BlockDriverState *bs, 
uint64_t *bitmap_table,
-        qcow2_free_clusters(bs, addr, s->cluster_size, QCOW2_DISCARD_OTHER);
+        qcow2_free_clusters(bs, addr, s->cluster_size, QCOW2_DISCARD_ALWAYS);
         bitmap_table[i] = 0;

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