[Qemu-devel] [PATCHv3 3/6] block: add a knob to disable multiwrite_merge

[Peter Lieven <pl@kamp.de>]

The block layer silently merges write requests since
commit 40b4f539. This patch adds a knob to disable
this feature as there has been some discussion lately
if multiwrite is a good idea at all and as it falsifies
benchmarks.

Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Max Reitz <mreitz@redhat.com>
---
 block.c                   |    9 +++++++++
 block/qapi.c              |    1 +
 hmp.c                     |    4 ++++
 include/block/block_int.h |    1 +
 qapi/block-core.json      |   10 +++++++++-
 qemu-options.hx           |    1 +
 qmp-commands.hx           |    2 ++
 7 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/block.c b/block.c
index f05ea0c..f3da5dd 100644
--- a/block.c
+++ b/block.c
@@ -884,6 +884,10 @@ static QemuOptsList bdrv_runtime_opts = {
             .name = "node-name",
             .type = QEMU_OPT_STRING,
             .help = "Node name of the block device node",
+        },{
+            .name = "write-merging",
+            .type = QEMU_OPT_BOOL,
+            .help = "enable write merging (default: true)",
         },
         { /* end of list */ }
     },
@@ -986,6 +990,7 @@ static int bdrv_open_common(BlockDriverState *bs, BlockDriverState *file,
     bs->opaque = g_malloc0(drv->instance_size);
 
     bs->enable_write_cache = !!(flags & BDRV_O_CACHE_WB);
+    bs->write_merging = qemu_opt_get_bool(opts, "write-merging", true);
 
     /* Open the image, either directly or using a protocol */
     if (drv->bdrv_file_open) {
@@ -4451,6 +4456,10 @@ static int multiwrite_merge(BlockDriverState *bs, BlockRequest *reqs,
 {
     int i, outidx;
 
+    if (!bs->write_merging) {
+        return num_reqs;
+    }
+
     // Sort requests by start sector
     qsort(reqs, num_reqs, sizeof(*reqs), &multiwrite_req_compare);
 
diff --git a/block/qapi.c b/block/qapi.c
index 28ea4db..9136705 100644
--- a/block/qapi.c
+++ b/block/qapi.c
@@ -59,6 +59,7 @@ BlockDeviceInfo *bdrv_block_device_info(BlockDriverState *bs)
 
     info->backing_file_depth = bdrv_get_backing_file_depth(bs);
     info->detect_zeroes = bs->detect_zeroes;
+    info->write_merging = bs->write_merging;
 
     if (bs->io_limits_enabled) {
         ThrottleConfig cfg;
diff --git a/hmp.c b/hmp.c
index 5741dfd..48b85dd 100644
--- a/hmp.c
+++ b/hmp.c
@@ -348,6 +348,10 @@ void hmp_info_block(Monitor *mon, const QDict *qdict)
                            BlockdevDetectZeroesOptions_lookup[info->value->inserted->detect_zeroes]);
         }
 
+        if (!info->value->inserted->write_merging) {
+            monitor_printf(mon, "    Write Merging:    off\n");
+        }
+
         if (info->value->inserted->bps
             || info->value->inserted->bps_rd
             || info->value->inserted->bps_wr
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 8898c6c..e3d382f 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -402,6 +402,7 @@ struct BlockDriverState {
 
     QDict *options;
     BlockdevDetectZeroesOptions detect_zeroes;
+    bool write_merging;
 
     /* The error object in use for blocking operations on backing_hd */
     Error *backing_blocker;
diff --git a/qapi/block-core.json b/qapi/block-core.json
index 2095f9e..74d1960 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -214,6 +214,8 @@
 #
 # @detect_zeroes: detect and optimize zero writes (Since 2.1)
 #
+# @write_merging: true if write merging is enabled (Since 2.2)
+#
 # @bps: total throughput limit in bytes per second is specified
 #
 # @bps_rd: read throughput limit in bytes per second is specified
@@ -250,6 +252,7 @@
             '*backing_file': 'str', 'backing_file_depth': 'int',
             'encrypted': 'bool', 'encryption_key_missing': 'bool',
             'detect_zeroes': 'BlockdevDetectZeroesOptions',
+            'write_merging': 'bool',
             'bps': 'int', 'bps_rd': 'int', 'bps_wr': 'int',
             'iops': 'int', 'iops_rd': 'int', 'iops_wr': 'int',
             'image': 'ImageInfo',
@@ -1185,6 +1188,10 @@
 #                 (default: false)
 # @detect-zeroes: #optional detect and optimize zero writes (Since 2.1)
 #                 (default: off)
+# @write-merging: #optional enable the merging of write requests
+#                 also known as multiwrite_merge (Since 2.2)
+#                 (default: true, but this might change in the future
+#                 depending on format/protocol/features used)
 #
 # Since: 1.7
 ##
@@ -1198,7 +1205,8 @@
             '*rerror': 'BlockdevOnError',
             '*werror': 'BlockdevOnError',
             '*read-only': 'bool',
-            '*detect-zeroes': 'BlockdevDetectZeroesOptions' } }
+            '*detect-zeroes': 'BlockdevDetectZeroesOptions',
+            '*write-merging': 'bool' } }
 
 ##
 # @BlockdevOptionsFile
diff --git a/qemu-options.hx b/qemu-options.hx
index 22cf3b9..d2f756f 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -432,6 +432,7 @@ DEF("drive", HAS_ARG, QEMU_OPTION_drive,
     "       [,werror=ignore|stop|report|enospc][,id=name][,aio=threads|native]\n"
     "       [,readonly=on|off][,copy-on-read=on|off]\n"
     "       [,discard=ignore|unmap][,detect-zeroes=on|off|unmap]\n"
+    "       [,write-merging=on|off]\n"
     "       [[,bps=b]|[[,bps_rd=r][,bps_wr=w]]]\n"
     "       [[,iops=i]|[[,iops_rd=r][,iops_wr=w]]]\n"
     "       [[,bps_max=bm]|[[,bps_rd_max=rm][,bps_wr_max=wm]]]\n"
diff --git a/qmp-commands.hx b/qmp-commands.hx
index 1abd619..2c20207 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -2104,6 +2104,7 @@ Each json-object contain the following:
          - "iops_size": I/O size when limiting by iops (json-int)
          - "detect_zeroes": detect and optimize zero writing (json-string)
              - Possible values: "off", "on", "unmap"
+         - "write_merging": enable merging of write requests (json-bool)
          - "image": the detail of the image, it is a json-object containing
             the following:
              - "filename": image file name (json-string)
@@ -2181,6 +2182,7 @@ Example:
                "iops_wr_max": 0,
                "iops_size": 0,
                "detect_zeroes": "on",
+               "write_merging": "true",
                "image":{
                   "filename":"disks/test.qcow2",
                   "format":"qcow2",
-- 
1.7.9.5