qcow2: Add QcowCache

This adds some new cache functions to qcow2 which can be used for caching
refcount blocks and L2 tables. When used with cache=writethrough they work
like the old caching code which is spread all over qcow2, so for this case we
have merely a cleanup.

The interesting case is with writeback caching (this includes cache=none) where
data isn't written to disk immediately but only kept in cache initially. This
leads to some form of metadata write batching which avoids the current "write
to refcount block, flush, write to L2 table" pattern for each single request
when a lot of cluster allocations happen. Instead, cache entries are only
written out if its required to maintain the right order. In the pure cluster
allocation case this means that all metadata updates for requests are done in
memory initially and on sync, first the refcount blocks are written to disk,
then fsync, then L2 tables.

This improves performance of scenarios with lots of cluster allocations
noticably (e.g. installation or after taking a snapshot).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/block/qcow2.h b/block/qcow2.h
index 5217bea8a2..e5473e1efe 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -78,6 +78,9 @@ typedef struct QCowSnapshot {
     uint64_t vm_clock_nsec;
 } QCowSnapshot;
 
+struct Qcow2Cache;
+typedef struct Qcow2Cache Qcow2Cache;
+
 typedef struct BDRVQcowState {
     int cluster_bits;
     int cluster_size;
@@ -215,4 +218,20 @@ int qcow2_snapshot_load_tmp(BlockDriverState *bs, const char *snapshot_name);
 void qcow2_free_snapshots(BlockDriverState *bs);
 int qcow2_read_snapshots(BlockDriverState *bs);
 
+/* qcow2-cache.c functions */
+Qcow2Cache *qcow2_cache_create(BlockDriverState *bs, int num_tables,
+    bool writethrough);
+int qcow2_cache_destroy(BlockDriverState* bs, Qcow2Cache *c);
+
+void qcow2_cache_entry_mark_dirty(Qcow2Cache *c, void *table);
+int qcow2_cache_flush(BlockDriverState *bs, Qcow2Cache *c);
+int qcow2_cache_set_dependency(BlockDriverState *bs, Qcow2Cache *c,
+    Qcow2Cache *dependency);
+
+int qcow2_cache_get(BlockDriverState *bs, Qcow2Cache *c, uint64_t offset,
+    void **table);
+int qcow2_cache_get_empty(BlockDriverState *bs, Qcow2Cache *c, uint64_t offset,
+    void **table);
+int qcow2_cache_put(BlockDriverState *bs, Qcow2Cache *c, void **table);
+
 #endif