[Qemu-devel] [PATCHv3] block: optimize zero writes with bdrv_write_zeroes

[Peter Lieven]

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 <address@hidden>
---
v2->v3: - moved parameter parsing to blockdev_init
        - added per device detect_zeroes status to 
          hmp (info block -v) and qmp (query-block) [Eric]
        - added support to enable detect-zeroes also
          for hot added devices [Eric]
        - added missing entry to qemu_common_drive_opts
        - fixed description of qemu_iovec_is_zero [Fam]

v1->v2: - added tests to commit message (Markus)
RFCv2->v1: - fixed paramter parsing strncmp -> strcmp (Eric)
           - fixed typo (choosen->chosen) (Eric)
           - added second example to commit msg

RFCv1->RFCv2: - add detect-zeroes=off|on|unmap knob to drive cmdline parameter
              - call zero detection only for format (bs->file != NULL)

 block.c                   |   11 ++++++++++
 block/qapi.c              |   11 ++++++++++
 blockdev.c                |   28 +++++++++++++++++++++++++
 hmp.c                     |    6 ++++++
 include/block/block_int.h |   12 +++++++++++
 include/qemu-common.h     |    1 +
 qapi-schema.json          |   50 +++++++++++++++++++++++++++++++--------------
 qemu-options.hx           |    6 ++++++
 qmp-commands.hx           |    3 +++
 util/iov.c                |   21 +++++++++++++++++++
 10 files changed, 134 insertions(+), 15 deletions(-)

diff --git a/block.c b/block.c
index b749d31..f27b35d 100644
--- a/block.c
+++ b/block.c
@@ -3244,6 +3244,17 @@ 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 != BDRV_DETECT_ZEROES_OFF &&
+        !(flags & BDRV_REQ_ZERO_WRITE) && bs->file &&
+        drv->bdrv_co_write_zeroes && qemu_iovec_is_zero(qiov)) {
+        flags |= BDRV_REQ_ZERO_WRITE;
+        /* if the device was not opened with discard=on the below flag
+         * is immediately cleared again in bdrv_co_do_write_zeroes */
+        if (bs->detect_zeroes == BDRV_DETECT_ZEROES_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) {
diff --git a/block/qapi.c b/block/qapi.c
index af11445..fbf66c2 100644
--- a/block/qapi.c
+++ b/block/qapi.c
@@ -51,6 +51,17 @@ BlockDeviceInfo *bdrv_block_device_info(BlockDriverState *bs)
 
     info->backing_file_depth = bdrv_get_backing_file_depth(bs);
 
+    switch (bs->detect_zeroes) {
+    case BDRV_DETECT_ZEROES_ON:
+        info->detect_zeroes = g_strdup("on");
+        break;
+    case BDRV_DETECT_ZEROES_UNMAP:
+        info->detect_zeroes = g_strdup("unmap");
+        break;
+    default:
+        info->detect_zeroes = g_strdup("off");
+    }
+
     if (bs->io_limits_enabled) {
         ThrottleConfig cfg;
         throttle_get_config(&bs->throttle_state, &cfg);
diff --git a/blockdev.c b/blockdev.c
index 7810e9f..96c11fd 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -288,6 +288,21 @@ static int parse_block_error_action(const char *buf, bool 
is_read, Error **errp)
     }
 }
 
+static BdrvDetectZeroes parse_detect_zeroes(const char *buf, Error **errp)
+{
+    if (!buf || !strcmp(buf, "off")) {
+        return BDRV_DETECT_ZEROES_OFF;
+    } else if (!strcmp(buf, "on")) {
+        return BDRV_DETECT_ZEROES_ON;
+    } else if (!strcmp(buf, "unmap")) {
+        return BDRV_DETECT_ZEROES_UNMAP;
+    } else {
+        error_setg(errp, "invalid value for detect-zeroes: %s",
+                   buf);
+    }
+    return BDRV_DETECT_ZEROES_OFF;
+}
+
 static bool check_throttle_config(ThrottleConfig *cfg, Error **errp)
 {
     if (throttle_conflicting(cfg)) {
@@ -324,6 +339,7 @@ static DriveInfo *blockdev_init(const char *file, QDict 
*bs_opts,
     QemuOpts *opts;
     const char *id;
     bool has_driver_specific_opts;
+    BdrvDetectZeroes detect_zeroes;
     BlockDriver *drv = NULL;
 
     /* Check common options by copying from bs_opts to opts, all other options
@@ -452,6 +468,13 @@ static DriveInfo *blockdev_init(const char *file, QDict 
*bs_opts,
         }
     }
 
+    detect_zeroes =
+        parse_detect_zeroes(qemu_opt_get(opts, "detect-zeroes"), &error);
+    if (error) {
+        error_propagate(errp, error);
+        goto early_err;
+    }
+
     /* init */
     dinfo = g_malloc0(sizeof(*dinfo));
     dinfo->id = g_strdup(qemu_opts_id(opts));
@@ -462,6 +485,7 @@ 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);
@@ -2455,6 +2479,10 @@ 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",
         },
         { /* end of list */ }
     },
diff --git a/hmp.c b/hmp.c
index ca869ba..b1942ed 100644
--- a/hmp.c
+++ b/hmp.c
@@ -336,6 +336,12 @@ void hmp_info_block(Monitor *mon, const QDict *qdict)
                            info->value->inserted->backing_file_depth);
         }
 
+        if (verbose) {
+            monitor_printf(mon,
+                           "    Detect zeroes:    %s\n",
+                           info->value->inserted->detect_zeroes);
+        }
+
         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 9ffcb69..7b9ca05 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -271,6 +271,17 @@ typedef struct BlockLimits {
 } BlockLimits;
 
 /*
+ * Different operation modes for automatic zero detection
+ * to speed the write operation up with bdrv_write_zeroes.
+ */
+typedef enum {
+    BDRV_DETECT_ZEROES_OFF   = 0x0,
+    BDRV_DETECT_ZEROES_ON    = 0x1,
+    /* also set the BDRV_MAY_UNMAP flag with bdrv_write_zeroes */
+    BDRV_DETECT_ZEROES_UNMAP = 0x2,
+} BdrvDetectZeroes;
+
+/*
  * Note: the function bdrv_append() copies and swaps contents of
  * BlockDriverStates, so if you add new fields to this struct, please
  * inspect bdrv_append() to determine if the new fields need to be
@@ -364,6 +375,7 @@ struct BlockDriverState {
     BlockJob *job;
 
     QDict *options;
+    BdrvDetectZeroes detect_zeroes;
 };
 
 int get_tmp_filename(char *filename, int size);
diff --git a/include/qemu-common.h b/include/qemu-common.h
index a998e8d..8e3d6eb 100644
--- a/include/qemu-common.h
+++ b/include/qemu-common.h
@@ -330,6 +330,7 @@ void qemu_iovec_concat(QEMUIOVector *dst,
 void qemu_iovec_concat_iov(QEMUIOVector *dst,
                            struct iovec *src_iov, unsigned int src_cnt,
                            size_t soffset, size_t sbytes);
+bool qemu_iovec_is_zero(QEMUIOVector *qiov);
 void qemu_iovec_destroy(QEMUIOVector *qiov);
 void qemu_iovec_reset(QEMUIOVector *qiov);
 size_t qemu_iovec_to_buf(QEMUIOVector *qiov, size_t offset,
diff --git a/qapi-schema.json b/qapi-schema.json
index 0b00427..5e3b4a89 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -937,6 +937,8 @@
 # @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
@@ -972,6 +974,7 @@
   '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': 'str',
             'bps': 'int', 'bps_rd': 'int', 'bps_wr': 'int',
             'iops': 'int', 'iops_rd': 'int', 'iops_wr': 'int',
             'image': 'ImageInfo',
@@ -4250,6 +4253,20 @@
   'data': [ 'ignore', 'unmap' ] }
 
 ##
+# @BlockdevDetectZeroesOptions
+#
+# Selects the operation mode for zero write detection.
+#
+# @off:      Disabled
+# @on:       Enabled
+# @unmap:    Enabled and even try to unmap blocks if possible
+#
+# Since: 2.1
+##
+{ 'enum': 'BlockdevDetectZeroesOptions',
+  'data': [ 'off', 'on', 'unmap' ] }
+
+##
 # @BlockdevAioOptions
 #
 # Selects the AIO backend to handle I/O requests
@@ -4301,20 +4318,22 @@
 # Options that are available for all block devices, independent of the block
 # driver.
 #
-# @driver:      block driver name
-# @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
-#               currently not allowed on any other level.
-# @node-name:   #optional the name of a block driver state node (Since 2.0)
-# @discard:     #optional discard-related options (default: ignore)
-# @cache:       #optional cache-related options
-# @aio:         #optional AIO backend (default: threads)
-# @rerror:      #optional how to handle read errors on the device
-#               (default: report)
-# @werror:      #optional how to handle write errors on the device
-#               (default: enospc)
-# @read-only:   #optional whether the block device should be read-only
-#               (default: false)
+# @driver:        block driver name
+# @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
+#                 currently not allowed on any other level.
+# @node-name:     #optional the name of a block driver state node (Since 2.0)
+# @discard:       #optional discard-related options (default: ignore)
+# @cache:         #optional cache-related options
+# @aio:           #optional AIO backend (default: threads)
+# @rerror:        #optional how to handle read errors on the device
+#                 (default: report)
+# @werror:        #optional how to handle write errors on the device
+#                 (default: enospc)
+# @read-only:     #optional whether the block device should be read-only
+#                 (default: false)
+# @detect-zeroes: #optional detect and optimize zero writes (Since 2.1)
+#                 (default: off)
 #
 # Since: 1.7
 ##
@@ -4327,7 +4346,8 @@
             '*aio': 'BlockdevAioOptions',
             '*rerror': 'BlockdevOnError',
             '*werror': 'BlockdevOnError',
-            '*read-only': 'bool' } }
+            '*read-only': 'bool',
+            '*detect-zeroes': 'BlockdevDetectZeroesOptions' } }
 
 ##
 # @BlockdevOptionsFile
diff --git a/qemu-options.hx b/qemu-options.hx
index 781af14..5ee94ea 100644
--- a/qemu-options.hx
+++ b/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 address@hidden
 @var{copy-on-read} is "on" or "off" and enables whether to copy read backing
 file sectors into the image file.
address@hidden address@hidden
address@hidden is "off", "on" or "unmap" and enables the automatic
+conversion of plain zero writes by the OS to driver specific optimized
+zero write commands. If "unmap" is chosen and @var{discard} is "on"
+a zero write may even be converted to an UNMAP operation.
 @end table
 
 By default, the @option{cache=writeback} mode is used. It will report data
diff --git a/qmp-commands.hx b/qmp-commands.hx
index ed3ab92..a535955 100644
--- a/qmp-commands.hx
+++ b/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 writes (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",
diff --git a/util/iov.c b/util/iov.c
index 6569b5a..f8c49a1 100644
--- a/util/iov.c
+++ b/util/iov.c
@@ -335,6 +335,27 @@ void qemu_iovec_concat(QEMUIOVector *dst,
     qemu_iovec_concat_iov(dst, src->iov, src->niov, soffset, sbytes);
 }
 
+/*
+ *  check if the contents of the iovecs are all zero
+ */
+bool qemu_iovec_is_zero(QEMUIOVector *qiov)
+{
+    int i;
+    for (i = 0; i < qiov->niov; i++) {
+        size_t offs = qiov->iov[i].iov_len & ~(4 * sizeof(long) - 1);
+        uint8_t *ptr = qiov->iov[i].iov_base;
+        if (offs && !buffer_is_zero(qiov->iov[i].iov_base, offs)) {
+            return false;
+        }
+        for (; offs < qiov->iov[i].iov_len; offs++) {
+            if (ptr[offs]) {
+                return false;
+            }
+        }
+    }
+    return true;
+}
+
 void qemu_iovec_destroy(QEMUIOVector *qiov)
 {
     assert(qiov->nalloc != -1);
-- 
1.7.9.5