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>
diff --git a/blockdev.c b/blockdev.c
index 78b927f..1cbcc1c 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -343,6 +343,7 @@
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 @@
}
}
+ 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 @@
}
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 @@
.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 */ }
},