[Qemu-devel] [PATCHv2] 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>
---
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                   |   39 ++++++++++++++++++++++++++++++++++++++-
 include/block/block_int.h |   12 ++++++++++++
 include/qemu-common.h     |    1 +
 qemu-options.hx           |    6 ++++++
 util/iov.c                |   21 +++++++++++++++++++++
 5 files changed, 78 insertions(+), 1 deletion(-)

diff --git a/block.c b/block.c
index acb70fd..3f61f76 100644
--- a/block.c
+++ b/block.c
@@ -817,6 +817,25 @@ static int bdrv_assign_node_name(BlockDriverState *bs,
     return 0;
 }
 
+static int bdrv_set_detect_zeroes(BlockDriverState *bs,
+                                  const char *detect_zeroes,
+                                  Error **errp)
+{
+    if (!detect_zeroes || !strcmp(detect_zeroes, "off")) {
+        bs->detect_zeroes = BDRV_DETECT_ZEROES_OFF;
+    } else if (!strcmp(detect_zeroes, "on")) {
+        bs->detect_zeroes = BDRV_DETECT_ZEROES_ON;
+    } else if (!strcmp(detect_zeroes, "unmap")) {
+        bs->detect_zeroes = BDRV_DETECT_ZEROES_UNMAP;
+    } else {
+        error_setg(errp, "invalid value for detect-zeroes: %s",
+                   detect_zeroes);
+        return -EINVAL;
+    }
+
+    return 0;
+}
+
 /*
  * Common part for opening disk images and files
  *
@@ -827,7 +846,7 @@ static int bdrv_open_common(BlockDriverState *bs, 
BlockDriverState *file,
 {
     int ret, open_flags;
     const char *filename;
-    const char *node_name = NULL;
+    const char *node_name = NULL, *detect_zeroes = NULL;
     Error *local_err = NULL;
 
     assert(drv != NULL);
@@ -855,6 +874,13 @@ static int bdrv_open_common(BlockDriverState *bs, 
BlockDriverState *file,
     }
     qdict_del(options, "node-name");
 
+    detect_zeroes = qdict_get_try_str(options, "detect-zeroes");
+    ret = bdrv_set_detect_zeroes(bs, detect_zeroes, errp);
+    if (ret < 0) {
+        return ret;
+    }
+    qdict_del(options, "detect-zeroes");
+
     /* bdrv_open() with directly using a protocol as drv. This layer is already
      * opened, so assign it to bs (while file becomes a closed 
BlockDriverState)
      * and return immediately. */
@@ -3179,6 +3205,17 @@ 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/include/block/block_int.h b/include/block/block_int.h
index cd5bc73..7a3013a 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
@@ -365,6 +376,7 @@ 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 c8a58a8..574da73 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/qemu-options.hx b/qemu-options.hx
index ee5437b..f824d9b 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -410,6 +410,7 @@ DEF("drive", HAS_ARG, QEMU_OPTION_drive,
     "       
[,cache=writethrough|writeback|none|directsync|unsafe][,format=f]\n"
     "       [,serial=s][,addr=A][,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"
@@ -470,6 +471,11 @@ 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/util/iov.c b/util/iov.c
index 6569b5a..0b17392 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 is 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