[PATCH v3 12/12] block/qcow2: automatically insert preallocate filter when on FUSE

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

vstorage has slow allocation, so this patch detect vstorage
(I hope, we don't use other FUSE filesystems) and inserts preallocate
filter between qcow2 node and its file child.

The following test executes more than 10 times faster
(43.2s -> 3.9s for me) with this patch. (/newssd3 is mount point of
vstorage, both cs and mds are on same ssd local ssd disk)

    IMG=/newssd3/z
    FILE_OPTS=file.filename=$IMG
    COUNT=15000
    CHUNK=64K
    CLUSTER=1M
    rm -f $IMG
    ./qemu-img create -f qcow2 -o cluster_size=$CLUSTER $IMG 1G
    ./qemu-img bench -c $COUNT -d 1 -s $CHUNK -w -t none -f qcow2 $IMG

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/qcow2.c | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/block/qcow2.c b/block/qcow2.c
index 6ad6bdc166..f56507158e 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1868,6 +1868,40 @@ static void coroutine_fn qcow2_open_entry(void *opaque)
     qemu_co_mutex_unlock(&s->lock);
 }
 
+static int qcow2_vz_insert_prealloc_filter(BlockDriverState *bs, Error **errp)
+{
+    QDict *options;
+    BlockDriverState *filter_bs;
+    Error *local_err = NULL;
+    int flags;
+
+    if (!bdrv_is_file_on_fuse(bs->file->bs)) {
+        /* Nothing to do */
+        return 0;
+    }
+
+    /* Assume it's a vstorage */
+    options = qdict_new();
+    qdict_put_str(options, "driver", "preallocate");
+    qdict_put_str(options, "file", bs->file->bs->node_name);
+    flags = bdrv_is_read_only(bs->file->bs) ? 0 : BDRV_O_RDWR;
+    filter_bs = bdrv_open(NULL, NULL, options, flags, errp);
+    if (!filter_bs) {
+        return -EINVAL;
+    }
+
+    bdrv_replace_node(bs->file->bs, filter_bs, &local_err);
+
+    /*
+     * On failure we want to remove filter_bs, on success it's referenced now by
+     * qcow2 node.
+     */
+    bdrv_unref(filter_bs);
+
+    error_propagate(errp, local_err);
+    return local_err ? -EINVAL : 0;
+}
+
 static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
                       Error **errp)
 {
@@ -1886,6 +1920,10 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
         return -EINVAL;
     }
 
+    if (qcow2_vz_insert_prealloc_filter(bs, errp) < 0) {
+        return -EINVAL;
+    }
+
     /* Initialise locks */
     qemu_co_mutex_init(&s->lock);
 
-- 
2.18.0