block: optimize zero writes with bdrv_write_zeroes

this patch tries to optimize zero write requests
by automatically using bdrv_write_zeroes if it is
supported by the format.

This significantly speeds up file system initialization and
should speed zero write test used to test backend storage
performance.

I ran the following 2 tests on my internal SSD with a
50G QCOW2 container and on an attached iSCSI storage.

a) mkfs.ext4 -E lazy_itable_init=0,lazy_journal_init=0 /dev/vdX

QCOW2         [off]     [on]     [unmap]
-----
runtime:       14secs    1.1secs  1.1secs
filesize:      937M      18M      18M

iSCSI         [off]     [on]     [unmap]
----
runtime:       9.3s      0.9s     0.9s

b) dd if=/dev/zero of=/dev/vdX bs=1M oflag=direct

QCOW2         [off]     [on]     [unmap]
-----
runtime:       246secs   18secs   18secs
filesize:      51G       192K     192K
throughput:    203M/s    2.3G/s   2.3G/s

iSCSI*        [off]     [on]     [unmap]
----
runtime:       8mins     45secs   33secs
throughput:    106M/s    1.2G/s   1.6G/s
allocated:     100%      100%     0%

* The storage was connected via an 1Gbit interface.
  It seems to internally handle writing zeroes
  via WRITESAME16 very fast.

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -3289,6 +3289,15 @@ static int coroutine_fn bdrv_aligned_pwritev(BlockDriverState *bs,
 
     ret = notifier_with_return_list_notify(&bs->before_write_notifiers, req);
 
+    if (!ret && bs->detect_zeroes != BLOCKDEV_DETECT_ZEROES_OPTIONS_OFF &&
+        !(flags & BDRV_REQ_ZERO_WRITE) && drv->bdrv_co_write_zeroes &&
+        qemu_iovec_is_zero(qiov)) {
+        flags |= BDRV_REQ_ZERO_WRITE;
+        if (bs->detect_zeroes == BLOCKDEV_DETECT_ZEROES_OPTIONS_UNMAP) {
+            flags |= BDRV_REQ_MAY_UNMAP;
+        }
+    }
+
     if (ret < 0) {
         /* Do nothing, write notifier decided to fail this request */
     } else if (flags & BDRV_REQ_ZERO_WRITE) {

block/qapi.c

@@ -50,6 +50,7 @@ BlockDeviceInfo *bdrv_block_device_info(BlockDriverState *bs)
     }
 
     info->backing_file_depth = bdrv_get_backing_file_depth(bs);
+    info->detect_zeroes = bs->detect_zeroes;
 
     if (bs->io_limits_enabled) {
         ThrottleConfig cfg;

blockdev.c

@@ -343,6 +343,7 @@ static DriveInfo *blockdev_init(const char *file, QDict *bs_opts,
     QemuOpts *opts;
     const char *id;
     bool has_driver_specific_opts;
+    BlockdevDetectZeroesOptions detect_zeroes;
     BlockDriver *drv = NULL;
 
     /* Check common options by copying from bs_opts to opts, all other options
@@ -471,6 +472,24 @@ static DriveInfo *blockdev_init(const char *file, QDict *bs_opts,
         }
     }
 
+    detect_zeroes =
+        parse_enum_option(BlockdevDetectZeroesOptions_lookup,
+                          qemu_opt_get(opts, "detect-zeroes"),
+                          BLOCKDEV_DETECT_ZEROES_OPTIONS_MAX,
+                          BLOCKDEV_DETECT_ZEROES_OPTIONS_OFF,
+                          &error);
+    if (error) {
+        error_propagate(errp, error);
+        goto early_err;
+    }
+
+    if (detect_zeroes == BLOCKDEV_DETECT_ZEROES_OPTIONS_UNMAP &&
+        !(bdrv_flags & BDRV_O_UNMAP)) {
+        error_setg(errp, "setting detect-zeroes to unmap is not allowed "
+                         "without setting discard operation to unmap");
+        goto early_err;
+    }
+
     /* init */
     dinfo = g_malloc0(sizeof(*dinfo));
     dinfo->id = g_strdup(qemu_opts_id(opts));
@@ -481,6 +500,7 @@ static DriveInfo *blockdev_init(const char *file, QDict *bs_opts,
     }
     dinfo->bdrv->open_flags = snapshot ? BDRV_O_SNAPSHOT : 0;
     dinfo->bdrv->read_only = ro;
+    dinfo->bdrv->detect_zeroes = detect_zeroes;
     dinfo->refcount = 1;
     if (serial != NULL) {
         dinfo->serial = g_strdup(serial);
@@ -2474,6 +2494,10 @@ QemuOptsList qemu_common_drive_opts = {
             .name = "copy-on-read",
             .type = QEMU_OPT_BOOL,
             .help = "copy read data from backing file into image file",
+        },{
+            .name = "detect-zeroes",
+            .type = QEMU_OPT_STRING,
+            .help = "try to optimize zero writes (off, on, unmap)",
         },
         { /* end of list */ }
     },

hmp.c

@@ -341,6 +341,11 @@ void hmp_info_block(Monitor *mon, const QDict *qdict)
                            info->value->inserted->backing_file_depth);
         }
 
+        if (info->value->inserted->detect_zeroes != BLOCKDEV_DETECT_ZEROES_OPTIONS_OFF) {
+            monitor_printf(mon, "    Detect zeroes:    %s\n",
+                           BlockdevDetectZeroesOptions_lookup[info->value->inserted->detect_zeroes]);
+        }
+
         if (info->value->inserted->bps
             || info->value->inserted->bps_rd
             || info->value->inserted->bps_wr

include/block/block_int.h

@@ -364,6 +364,7 @@ struct BlockDriverState {
     BlockJob *job;
 
     QDict *options;
+    BlockdevDetectZeroesOptions detect_zeroes;
 };
 
 int get_tmp_filename(char *filename, int size);

qapi-schema.json

@@ -942,6 +942,8 @@
 # @encryption_key_missing: true if the backing device is encrypted but an
 #                          valid encryption key is missing
 #
+# @detect_zeroes: detect and optimize zero writes (Since 2.1)
+#
 # @bps: total throughput limit in bytes per second is specified
 #
 # @bps_rd: read throughput limit in bytes per second is specified
@@ -977,6 +979,7 @@
   'data': { 'file': 'str', '*node-name': 'str', 'ro': 'bool', 'drv': 'str',
             '*backing_file': 'str', 'backing_file_depth': 'int',
             'encrypted': 'bool', 'encryption_key_missing': 'bool',
+            'detect_zeroes': 'BlockdevDetectZeroesOptions',
             'bps': 'int', 'bps_rd': 'int', 'bps_wr': 'int',
             'iops': 'int', 'iops_rd': 'int', 'iops_wr': 'int',
             'image': 'ImageInfo',
@@ -4254,6 +4257,22 @@
 { 'enum': 'BlockdevDiscardOptions',
   'data': [ 'ignore', 'unmap' ] }
 
+##
+# @BlockdevDetectZeroesOptions
+#
+# Describes the operation mode for the automatic conversion of plain
+# zero writes by the OS to driver specific optimized zero write commands.
+#
+# @off:      Disabled (default)
+# @on:       Enabled
+# @unmap:    Enabled and even try to unmap blocks if possible. This requires
+#            also that @BlockdevDiscardOptions is set to unmap for this device.
+#
+# Since: 2.1
+##
+{ 'enum': 'BlockdevDetectZeroesOptions',
+  'data': [ 'off', 'on', 'unmap' ] }
+
 ##
 # @BlockdevAioOptions
 #
@@ -4306,20 +4325,22 @@
 # Options that are available for all block devices, independent of the block
 # driver.
 #
-# @driver:      block driver name
+# @driver:        block driver name
-# @id:          #optional id by which the new block device can be referred to.
+# @id:            #optional id by which the new block device can be referred to.
-#               This is a required option on the top level of blockdev-add, and
+#                 This is a required option on the top level of blockdev-add, and
-#               currently not allowed on any other level.
+#                 currently not allowed on any other level.
-# @node-name:   #optional the name of a block driver state node (Since 2.0)
+# @node-name:     #optional the name of a block driver state node (Since 2.0)
-# @discard:     #optional discard-related options (default: ignore)
+# @discard:       #optional discard-related options (default: ignore)
-# @cache:       #optional cache-related options
+# @cache:         #optional cache-related options
-# @aio:         #optional AIO backend (default: threads)
+# @aio:           #optional AIO backend (default: threads)
-# @rerror:      #optional how to handle read errors on the device
+# @rerror:        #optional how to handle read errors on the device
-#               (default: report)
+#                 (default: report)
-# @werror:      #optional how to handle write errors on the device
+# @werror:        #optional how to handle write errors on the device
-#               (default: enospc)
+#                 (default: enospc)
-# @read-only:   #optional whether the block device should be read-only
+# @read-only:     #optional whether the block device should be read-only
-#               (default: false)
+#                 (default: false)
+# @detect-zeroes: #optional detect and optimize zero writes (Since 2.1)
+#                 (default: off)
 #
 # Since: 1.7
 ##
@@ -4332,7 +4353,8 @@
             '*aio': 'BlockdevAioOptions',
             '*rerror': 'BlockdevOnError',
             '*werror': 'BlockdevOnError',
-            '*read-only': 'bool' } }
+            '*read-only': 'bool',
+            '*detect-zeroes': 'BlockdevDetectZeroesOptions' } }
 
 ##
 # @BlockdevOptionsFile

qemu-options.hx

@@ -414,6 +414,7 @@ DEF("drive", HAS_ARG, QEMU_OPTION_drive,
     "       [,serial=s][,addr=A][,rerror=ignore|stop|report]\n"
     "       [,werror=ignore|stop|report|enospc][,id=name][,aio=threads|native]\n"
     "       [,readonly=on|off][,copy-on-read=on|off]\n"
+    "       [,detect-zeroes=on|off|unmap]\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"
@@ -475,6 +476,11 @@ Open drive @option{file} as read-only. Guest write attempts will fail.
 @item copy-on-read=@var{copy-on-read}
 @var{copy-on-read} is "on" or "off" and enables whether to copy read backing
 file sectors into the image file.
+@item detect-zeroes=@var{detect-zeroes}
+@var{detect-zeroes} is "off", "on" or "unmap" and enables the automatic
+conversion of plain zero writes by the OS to driver specific optimized
+zero write commands. You may even choose "unmap" if @var{discard} is set
+to "unmap" to allow a zero write to be converted to an UNMAP operation.
 @end table
 
 By default, the @option{cache=writeback} mode is used. It will report data

qmp-commands.hx

@@ -2032,6 +2032,8 @@ Each json-object contain the following:
          - "iops_rd_max":  read I/O operations max (json-int)
          - "iops_wr_max":  write I/O operations max (json-int)
          - "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"
          - "image": the detail of the image, it is a json-object containing
             the following:
              - "filename": image file name (json-string)
@@ -2108,6 +2110,7 @@ Example:
                "iops_rd_max": 0,
                "iops_wr_max": 0,
                "iops_size": 0,
+               "detect_zeroes": "on",
                "image":{
                   "filename":"disks/test.qcow2",
                   "format":"qcow2",

tests/qemu-iotests/067.out

@@ -6,7 +6,7 @@ Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728
 Testing: -drive file=TEST_DIR/t.qcow2,format=qcow2,if=none,id=disk -device virtio-blk-pci,drive=disk,id=virtio0
 QMP_VERSION
 {"return": {}}
-{"return": [{"io-status": "ok", "device": "disk", "locked": false, "removable": false, "inserted": {"iops_rd": 0, "image": {"virtual-size": 134217728, "filename": "TEST_DIR/t.qcow2", "cluster-size": 65536, "format": "qcow2", "actual-size": SIZE, "format-specific": {"type": "qcow2", "data": {"compat": "1.1", "lazy-refcounts": false}}, "dirty-flag": false}, "iops_wr": 0, "ro": false, "backing_file_depth": 0, "drv": "qcow2", "iops": 0, "bps_wr": 0, "encrypted": false, "bps": 0, "bps_rd": 0, "file": "TEST_DIR/t.qcow2", "encryption_key_missing": false}, "type": "unknown"}, {"io-status": "ok", "device": "ide1-cd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "floppy0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "sd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}]}
+{"return": [{"io-status": "ok", "device": "disk", "locked": false, "removable": false, "inserted": {"iops_rd": 0, "detect_zeroes": "off", "image": {"virtual-size": 134217728, "filename": "TEST_DIR/t.qcow2", "cluster-size": 65536, "format": "qcow2", "actual-size": SIZE, "format-specific": {"type": "qcow2", "data": {"compat": "1.1", "lazy-refcounts": false}}, "dirty-flag": false}, "iops_wr": 0, "ro": false, "backing_file_depth": 0, "drv": "qcow2", "iops": 0, "bps_wr": 0, "encrypted": false, "bps": 0, "bps_rd": 0, "file": "TEST_DIR/t.qcow2", "encryption_key_missing": false}, "type": "unknown"}, {"io-status": "ok", "device": "ide1-cd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "floppy0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "sd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}]}
 {"return": {}}
 {"return": {}}
 {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "DEVICE_DELETED", "data": {"path": "/machine/peripheral/virtio0/virtio-backend"}}
@@ -24,7 +24,7 @@ QMP_VERSION
 Testing: -drive file=TEST_DIR/t.qcow2,format=qcow2,if=none,id=disk
 QMP_VERSION
 {"return": {}}
-{"return": [{"device": "disk", "locked": false, "removable": true, "inserted": {"iops_rd": 0, "image": {"virtual-size": 134217728, "filename": "TEST_DIR/t.qcow2", "cluster-size": 65536, "format": "qcow2", "actual-size": SIZE, "format-specific": {"type": "qcow2", "data": {"compat": "1.1", "lazy-refcounts": false}}, "dirty-flag": false}, "iops_wr": 0, "ro": false, "backing_file_depth": 0, "drv": "qcow2", "iops": 0, "bps_wr": 0, "encrypted": false, "bps": 0, "bps_rd": 0, "file": "TEST_DIR/t.qcow2", "encryption_key_missing": false}, "tray_open": false, "type": "unknown"}, {"io-status": "ok", "device": "ide1-cd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "floppy0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "sd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}]}
+{"return": [{"device": "disk", "locked": false, "removable": true, "inserted": {"iops_rd": 0, "detect_zeroes": "off", "image": {"virtual-size": 134217728, "filename": "TEST_DIR/t.qcow2", "cluster-size": 65536, "format": "qcow2", "actual-size": SIZE, "format-specific": {"type": "qcow2", "data": {"compat": "1.1", "lazy-refcounts": false}}, "dirty-flag": false}, "iops_wr": 0, "ro": false, "backing_file_depth": 0, "drv": "qcow2", "iops": 0, "bps_wr": 0, "encrypted": false, "bps": 0, "bps_rd": 0, "file": "TEST_DIR/t.qcow2", "encryption_key_missing": false}, "tray_open": false, "type": "unknown"}, {"io-status": "ok", "device": "ide1-cd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "floppy0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "sd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}]}
 {"return": {}}
 {"return": {}}
 {"return": {}}
@@ -44,7 +44,7 @@ Testing:
 QMP_VERSION
 {"return": {}}
 {"return": "OK\r\n"}
-{"return": [{"io-status": "ok", "device": "ide1-cd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "floppy0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "sd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "disk", "locked": false, "removable": true, "inserted": {"iops_rd": 0, "image": {"virtual-size": 134217728, "filename": "TEST_DIR/t.qcow2", "cluster-size": 65536, "format": "qcow2", "actual-size": SIZE, "format-specific": {"type": "qcow2", "data": {"compat": "1.1", "lazy-refcounts": false}}, "dirty-flag": false}, "iops_wr": 0, "ro": false, "backing_file_depth": 0, "drv": "qcow2", "iops": 0, "bps_wr": 0, "encrypted": false, "bps": 0, "bps_rd": 0, "file": "TEST_DIR/t.qcow2", "encryption_key_missing": false}, "tray_open": false, "type": "unknown"}]}
+{"return": [{"io-status": "ok", "device": "ide1-cd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "floppy0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "sd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "disk", "locked": false, "removable": true, "inserted": {"iops_rd": 0, "detect_zeroes": "off", "image": {"virtual-size": 134217728, "filename": "TEST_DIR/t.qcow2", "cluster-size": 65536, "format": "qcow2", "actual-size": SIZE, "format-specific": {"type": "qcow2", "data": {"compat": "1.1", "lazy-refcounts": false}}, "dirty-flag": false}, "iops_wr": 0, "ro": false, "backing_file_depth": 0, "drv": "qcow2", "iops": 0, "bps_wr": 0, "encrypted": false, "bps": 0, "bps_rd": 0, "file": "TEST_DIR/t.qcow2", "encryption_key_missing": false}, "tray_open": false, "type": "unknown"}]}
 {"return": {}}
 {"return": {}}
 {"return": {}}
@@ -64,14 +64,14 @@ Testing:
 QMP_VERSION
 {"return": {}}
 {"return": {}}
-{"return": [{"io-status": "ok", "device": "ide1-cd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "floppy0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "sd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "disk", "locked": false, "removable": true, "inserted": {"iops_rd": 0, "image": {"virtual-size": 134217728, "filename": "TEST_DIR/t.qcow2", "cluster-size": 65536, "format": "qcow2", "actual-size": SIZE, "format-specific": {"type": "qcow2", "data": {"compat": "1.1", "lazy-refcounts": false}}, "dirty-flag": false}, "iops_wr": 0, "ro": false, "backing_file_depth": 0, "drv": "qcow2", "iops": 0, "bps_wr": 0, "encrypted": false, "bps": 0, "bps_rd": 0, "file": "TEST_DIR/t.qcow2", "encryption_key_missing": false}, "tray_open": false, "type": "unknown"}]}
+{"return": [{"io-status": "ok", "device": "ide1-cd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "floppy0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "sd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "disk", "locked": false, "removable": true, "inserted": {"iops_rd": 0, "detect_zeroes": "off", "image": {"virtual-size": 134217728, "filename": "TEST_DIR/t.qcow2", "cluster-size": 65536, "format": "qcow2", "actual-size": SIZE, "format-specific": {"type": "qcow2", "data": {"compat": "1.1", "lazy-refcounts": false}}, "dirty-flag": false}, "iops_wr": 0, "ro": false, "backing_file_depth": 0, "drv": "qcow2", "iops": 0, "bps_wr": 0, "encrypted": false, "bps": 0, "bps_rd": 0, "file": "TEST_DIR/t.qcow2", "encryption_key_missing": false}, "tray_open": false, "type": "unknown"}]}
 {"return": {}}
 {"return": {}}
 {"return": {}}
 {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "DEVICE_DELETED", "data": {"path": "/machine/peripheral/virtio0/virtio-backend"}}
 {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "DEVICE_DELETED", "data": {"device": "virtio0", "path": "/machine/peripheral/virtio0"}}
 {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "RESET"}
-{"return": [{"io-status": "ok", "device": "ide1-cd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "floppy0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "sd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"io-status": "ok", "device": "disk", "locked": false, "removable": true, "inserted": {"iops_rd": 0, "image": {"virtual-size": 134217728, "filename": "TEST_DIR/t.qcow2", "cluster-size": 65536, "format": "qcow2", "actual-size": SIZE, "format-specific": {"type": "qcow2", "data": {"compat": "1.1", "lazy-refcounts": false}}, "dirty-flag": false}, "iops_wr": 0, "ro": false, "backing_file_depth": 0, "drv": "qcow2", "iops": 0, "bps_wr": 0, "encrypted": false, "bps": 0, "bps_rd": 0, "file": "TEST_DIR/t.qcow2", "encryption_key_missing": false}, "tray_open": false, "type": "unknown"}]}
+{"return": [{"io-status": "ok", "device": "ide1-cd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "floppy0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"device": "sd0", "locked": false, "removable": true, "tray_open": false, "type": "unknown"}, {"io-status": "ok", "device": "disk", "locked": false, "removable": true, "inserted": {"iops_rd": 0, "detect_zeroes": "off", "image": {"virtual-size": 134217728, "filename": "TEST_DIR/t.qcow2", "cluster-size": 65536, "format": "qcow2", "actual-size": SIZE, "format-specific": {"type": "qcow2", "data": {"compat": "1.1", "lazy-refcounts": false}}, "dirty-flag": false}, "iops_wr": 0, "ro": false, "backing_file_depth": 0, "drv": "qcow2", "iops": 0, "bps_wr": 0, "encrypted": false, "bps": 0, "bps_rd": 0, "file": "TEST_DIR/t.qcow2", "encryption_key_missing": false}, "tray_open": false, "type": "unknown"}]}
 {"return": {}}
 {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "SHUTDOWN"}
 {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "DEVICE_TRAY_MOVED", "data": {"device": "ide1-cd0", "tray-open": true}}