[Qemu-devel] [PATCH] block: add a knob to disable multiwrite_merge

[Peter Lieven]

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 <address@hidden>
---
 block.c                   |    4 ++++
 block/qapi.c              |    1 +
 blockdev.c                |    7 +++++++
 hmp.c                     |    4 ++++
 include/block/block_int.h |    1 +
 qapi/block-core.json      |   10 +++++++++-
 qemu-options.hx           |    1 +
 qmp-commands.hx           |    2 ++
 8 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/block.c b/block.c
index 27533f3..1658a72 100644
--- a/block.c
+++ b/block.c
@@ -4531,6 +4531,10 @@ 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 9733ebd..02251dd 100644
--- a/block/qapi.c
+++ b/block/qapi.c
@@ -58,6 +58,7 @@ 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/blockdev.c b/blockdev.c
index e595910..13e47b8 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -378,6 +378,7 @@ static DriveInfo *blockdev_init(const char *file, QDict 
*bs_opts,
     const char *id;
     bool has_driver_specific_opts;
     BlockdevDetectZeroesOptions detect_zeroes;
+    bool write_merging;
     BlockDriver *drv = NULL;
 
     /* Check common options by copying from bs_opts to opts, all other options
@@ -405,6 +406,7 @@ static DriveInfo *blockdev_init(const char *file, QDict 
*bs_opts,
     snapshot = qemu_opt_get_bool(opts, "snapshot", 0);
     ro = qemu_opt_get_bool(opts, "read-only", 0);
     copy_on_read = qemu_opt_get_bool(opts, "copy-on-read", false);
+    write_merging = qemu_opt_get_bool(opts, "write-merging", true);
 
     if ((buf = qemu_opt_get(opts, "discard")) != NULL) {
         if (bdrv_parse_discard_flags(buf, &bdrv_flags) != 0) {
@@ -530,6 +532,7 @@ static DriveInfo *blockdev_init(const char *file, QDict 
*bs_opts,
     bs->open_flags = snapshot ? BDRV_O_SNAPSHOT : 0;
     bs->read_only = ro;
     bs->detect_zeroes = detect_zeroes;
+    bs->write_merging = write_merging;
 
     bdrv_set_on_error(bs, on_read_error, on_write_error);
 
@@ -2746,6 +2749,10 @@ QemuOptsList qemu_common_drive_opts = {
             .name = "detect-zeroes",
             .type = QEMU_OPT_STRING,
             .help = "try to optimize zero writes (off, on, unmap)",
+        },{
+            .name = "write-merging",
+            .type = QEMU_OPT_BOOL,
+            .help = "enable write merging (default: true)",
         },
         { /* end of list */ }
     },
diff --git a/hmp.c b/hmp.c
index 63d7686..8d6ad0b 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 8d86a6c..39bbde2 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -407,6 +407,7 @@ 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 8f7089e..4931bd9 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',
@@ -1180,6 +1183,10 @@
 #                 (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
 ##
@@ -1193,7 +1200,8 @@
             '*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..529a563 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 multiwrite_merge feature (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