[Qemu-devel] [RFC 3/3] qcow2: Evaluate rotational info

[Max Reitz <mreitz@redhat.com>]

The new handle_alloc_space() function only speeds up the allocation of
new ranges on solid-state drives.  We should skip it if we know that the
file is stored on a rotating disk.

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/qcow2.h |  3 +++
 block/qcow2.c | 34 ++++++++++++++++++++++++++++++----
 2 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/block/qcow2.h b/block/qcow2.h
index fc1b0d3c1e..5052ab187f 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -359,6 +359,9 @@ typedef struct BDRVQcow2State {
 
     bool metadata_preallocation_checked;
     bool metadata_preallocation;
+
+    /* True if the image is stored on a rotating disk */
+    bool optimize_for_rotating;
 } BDRVQcow2State;
 
 typedef struct Qcow2COWRegion {
diff --git a/block/qcow2.c b/block/qcow2.c
index 14f914117f..b4df6d5085 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1197,6 +1197,22 @@ static int qcow2_update_options(BlockDriverState *bs, QDict *options,
     return ret;
 }
 
+static void qcow2_update_rotational_info(BlockDriverState *bs)
+{
+    BDRVQcow2State *s = bs->opaque;
+    BlockDriverInfo file_bdi;
+
+    s->optimize_for_rotating = false;
+
+    if (bdrv_get_info(bs->file->bs, &file_bdi) < 0) {
+        return;
+    }
+
+    s->optimize_for_rotating =
+        file_bdi.has_rotational_info &&
+        file_bdi.rotational_info == IMAGE_ROTATIONAL_INFO_ROTATING;
+}
+
 /* Called with s->lock held.  */
 static int coroutine_fn qcow2_do_open(BlockDriverState *bs, QDict *options,
                                       int flags, Error **errp)
@@ -1461,6 +1477,8 @@ static int coroutine_fn qcow2_do_open(BlockDriverState *bs, QDict *options,
         }
     }
 
+    qcow2_update_rotational_info(bs);
+
     /* Parse driver-specific options */
     ret = qcow2_update_options(bs, options, flags, errp);
     if (ret < 0) {
@@ -1829,6 +1847,8 @@ static void qcow2_reopen_commit(BDRVReopenState *state)
 {
     qcow2_update_options_commit(state->bs, state->opaque);
     g_free(state->opaque);
+
+    qcow2_update_rotational_info(state->bs);
 }
 
 static void qcow2_reopen_abort(BDRVReopenState *state)
@@ -2297,10 +2317,16 @@ static coroutine_fn int qcow2_co_pwritev(BlockDriverState *bs, uint64_t offset,
             qemu_iovec_add(&hd_qiov, cluster_data, cur_bytes);
         }
 
-        /* Try to efficiently initialize the physical space with zeroes */
-        ret = handle_alloc_space(bs, l2meta);
-        if (ret < 0) {
-            goto out_unlocked;
+        /*
+         * Try to efficiently initialize the physical space with zeroes.
+         * This incurs a performance penalty on rotating disks, so
+         * avoid doing it there.
+         */
+        if (!s->optimize_for_rotating) {
+            ret = handle_alloc_space(bs, l2meta);
+            if (ret < 0) {
+                goto out_unlocked;
+            }
         }
 
         /* If we need to do COW, check if it's possible to merge the
-- 
2.21.0