Currently the order is this (during cow since it's the interesting case):
1. Decrement refcount of old clusters
2. Increment refcount for newly allocated clusters
3. Copy content of old sectors that will not be rewritten
4. Update L2 table with pointers to new clusters
5. Write guest data into new clusters (asynchronously)
There are several problems with this order. The first one is that if qemu
crashes (or killed or host reboots) after new clusters are linked into L2
table but before user data is written there, then on the next reboot guest
will find neither old data nor new one in those sectors and this is not
what gust expects even when journaling file system is in use. The other
problem is that if qemu is killed between steps 1 and 4 then refcount
of old cluster will be incorrect and may cause snapshot corruption.
The patch change the order to be like this:
1. Increment refcount for newly allocated clusters
2. Write guest data into new clusters (asynchronously)
3. Copy content of old sectors that were not rewritten
4. Update L2 table with pointers to new clusters
5. Decrement refcount of old clusters
Unexpected crash may cause cluster leakage, but guest data should be safe.
Signed-off-by: Gleb Natapov <address@hidden>
---
block-qcow2.c | 155 +++++++++++++++++++++++++++++++++------------------------
1 files changed, 91 insertions(+), 64 deletions(-)
diff --git a/block-qcow2.c b/block-qcow2.c
index 0771281..c600517 100644
--- a/block-qcow2.c
+++ b/block-qcow2.c
@@ -852,6 +852,69 @@ static uint64_t
alloc_compressed_cluster_offset(BlockDriverState *bs,
return cluster_offset;
}
+typedef struct QCowL2Meta
+{
+ uint64_t offset;
+ int n_start;
+ int nb_available;
+ int nb_clusters;
+} QCowL2Meta;
+
+static int alloc_cluster_link_l2(BlockDriverState *bs, uint64_t cluster_offset,
+ QCowL2Meta *m)
+{
+ BDRVQcowState *s = bs->opaque;
+ int i, j = 0, l2_index, ret;
+ uint64_t *old_cluster, start_sect, l2_offset, *l2_table;
+
+ if (m->nb_clusters == 0)
+ return 0;
+
+ if (!(old_cluster = qemu_malloc(m->nb_clusters * sizeof(uint64_t))))
+ return -ENOMEM;