[Qemu-devel] [PULL 3/9] block/stream: use BDRV_REQ_PREFETCH

[Eric Blake <eblake@redhat.com>]

From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

This helps to avoid extra io, allocations and memory copying.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20190725100550.33801-3-vsementsov@virtuozzo.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
[eblake: fix comment grammar]
Signed-off-by: Eric Blake <eblake@redhat.com>
---
 block/stream.c | 24 +++++++++---------------
 1 file changed, 9 insertions(+), 15 deletions(-)

diff --git a/block/stream.c b/block/stream.c
index 6ac1e7bec42c..0d3a6ac7c3f7 100644
--- a/block/stream.c
+++ b/block/stream.c
@@ -22,11 +22,11 @@

 enum {
     /*
-     * Size of data buffer for populating the image file.  This should be large
-     * enough to process multiple clusters in a single call, so that populating
-     * contiguous regions of the image is efficient.
+     * Maximum chunk size to feed to copy-on-read.  This should be
+     * large enough to process multiple clusters in a single call, so
+     * that populating contiguous regions of the image is efficient.
      */
-    STREAM_BUFFER_SIZE = 512 * 1024, /* in bytes */
+    STREAM_CHUNK = 512 * 1024, /* in bytes */
 };

 typedef struct StreamBlockJob {
@@ -39,13 +39,12 @@ typedef struct StreamBlockJob {
 } StreamBlockJob;

 static int coroutine_fn stream_populate(BlockBackend *blk,
-                                        int64_t offset, uint64_t bytes,
-                                        void *buf)
+                                        int64_t offset, uint64_t bytes)
 {
     assert(bytes < SIZE_MAX);

-    /* Copy-on-read the unallocated clusters */
-    return blk_co_pread(blk, offset, bytes, buf, BDRV_REQ_COPY_ON_READ);
+    return blk_co_preadv(blk, offset, bytes, NULL,
+                         BDRV_REQ_COPY_ON_READ | BDRV_REQ_PREFETCH);
 }

 static void stream_abort(Job *job)
@@ -117,7 +116,6 @@ static int coroutine_fn stream_run(Job *job, Error **errp)
     int error = 0;
     int ret = 0;
     int64_t n = 0; /* bytes */
-    void *buf;

     if (bs == s->bottom) {
         /* Nothing to stream */
@@ -130,8 +128,6 @@ static int coroutine_fn stream_run(Job *job, Error **errp)
     }
     job_progress_set_remaining(&s->common.job, len);

-    buf = qemu_blockalign(bs, STREAM_BUFFER_SIZE);
-
     /* Turn on copy-on-read for the whole block device so that guest read
      * requests help us make progress.  Only do this when copying the entire
      * backing chain since the copy-on-read operation does not take base into
@@ -154,7 +150,7 @@ static int coroutine_fn stream_run(Job *job, Error **errp)

         copy = false;

-        ret = bdrv_is_allocated(bs, offset, STREAM_BUFFER_SIZE, &n);
+        ret = bdrv_is_allocated(bs, offset, STREAM_CHUNK, &n);
         if (ret == 1) {
             /* Allocated in the top, no need to copy.  */
         } else if (ret >= 0) {
@@ -171,7 +167,7 @@ static int coroutine_fn stream_run(Job *job, Error **errp)
         }
         trace_stream_one_iteration(s, offset, n, ret);
         if (copy) {
-            ret = stream_populate(blk, offset, n, buf);
+            ret = stream_populate(blk, offset, n);
         }
         if (ret < 0) {
             BlockErrorAction action =
@@ -202,8 +198,6 @@ static int coroutine_fn stream_run(Job *job, Error **errp)
         bdrv_disable_copy_on_read(bs);
     }

-    qemu_vfree(buf);
-
     /* Do not remove the backing file if an error was there but ignored. */
     return error;
 }
-- 
2.20.1