[RFC PATCH v2 0/8] fallocate: introduce FALLOC_FL_WRITE_ZEROES flag

Zhang Yi
2025-01-15T13:02:37Z

From: Zhang Yi <yi.zhang@huawei.com>

Changes since v1:
- Switch to add a new write zeroes operation, FALLOC_FL_WRITE_ZEROES,
in fallocate, instead of just adding a supported flag to
FALLOC_FL_ZERO_RANGE.
- Introduce a new flag BLK_FEAT_WRITE_ZEROES_UNMAP to the block
device's queue limit features, and implement it on SCSI sd driver,
NVMe SSD driver and dm driver.
- Implement FALLOC_FL_WRITE_ZEROES on both the ext4 filesystem and
block device (bdev).

v1: https://lore.kernel.org/linux-fsdevel/20241228014522.2395187-1-yi.zhang@huaweicloud.com/

Currently, we can use the fallocate command to quickly create a
pre-allocated file. However, on most filesystems, such as ext4 and XFS,
fallocate create pre-allocation blocks in an unwritten state, and the
FALLOC_FL_ZERO_RANGE flag also behaves similarly. The extent state must
be converted to a written state when the user writes data into this
range later, which can trigger numerous metadata changes and consequent
journal I/O. This may leads to significant write amplification and
performance degradation in synchronous write mode. Therefore, we need a
method to create a pre-allocated file with written extents that can be
used for pure overwriting. At the monent, the only method available is
to create an empty file and write zero data into it (for example, using
'dd' with a large block size). However, this method is slow and consumes
a considerable amount of disk bandwidth, we must pre-allocate files in
advance but cannot add pre-allocated files while user business services
are running.

Fortunately, with the development and more and more widely used of
flash-based storage devices, we can efficiently write zeros to SSDs
using the unmap write zeroes command if the devices do not write
physical zeroes to the media. For example, if SCSI SSDs support the
UMMAP bit or NVMe SSDs support the DEAC bit[1], the write zeroes command
does not write actual data to the device, instead, NVMe converts the
zeroed range to a deallocated state, which works fast and consumes
almost no disk write bandwidth. Consequently, this feature can provide
us with a faster method for creating pre-allocated files with written
extents and zeroed data.

This series aims to implement this by:
1. Introduce a new feature BLK_FEAT_WRITE_ZEROES_UNMAP to the block
device queue limit features, which indicates whether the storage is
device explicitly supports the unmapped write zeroes command. This
flag should be set to 1 by the driver it the attached disk supports
this command. Users can check this flag by querying:

/sys/block/<disk>/queue/write_zeroes_unmap

2. Introduce a new flag FALLOC_FL_FORCE_ZERO into the fallocate,
filesystems with this operaion should allocate written extents and
issuing zeroes to the range of the device. If the device supports
unmap write zeroes command, the zeroing can be accelerated, if not,
we currently still allow to fall back to submit zeroes data. Users
can verify if the device supports the unmap write zeroes command and
then decide whether to use it.

I initially implemented the BLK_FEAT_WRITE_ZEROES_UNMAP flag for SCSI
and NVMe drivers, and I also added the FALLOC_FL_FORCE_ZERO flag for
ext4 and block devices. Any comments are welcome. Once the kernel
changes are finalized, I will do comprehensive tests, and update the
man page documentation, as well as the corresponding user-mode tools.

NOTE: this series is based on my ext4 fallocate refactor series[2] which
hasn't been merged to the mainline yet.

I've briefly modified xfs_io and fallocate tool in util-linux[3], and
tested performance with this series on ext4 filesystem on my machine
with an Intel Xeon Gold 6248R CPU, a 7TB KCD61LUL7T68 NVMe SSD which
supports unmap write zeroes command with the Deallocated state and the
DEAC bit. Feel free to give it a try.

0. Ensure the NVMe device supports WRITE_ZERO command.

$ cat /sys/block/nvme5n1/queue/write_zeroes_max_bytes
8388608
$ nvme id-ns -H /dev/nvme5n1 | grep -i -A 3 "dlfeat"
dlfeat : 25
[4:4] : 0x1 Guard Field of Deallocated Logical Blocks is set to CRC
of The Value Read
[3:3] : 0x1 Deallocate Bit in the Write Zeroes Command is Supported
[2:0] : 0x1 Bytes Read From a Deallocated Logical Block and its
Metadata are 0x00

1. Compare 'dd' and fallocate with force zero range, the zero range is
significantly faster than 'dd'.

a) Create a 1GB zeroed file.
$ dd if=/dev/zero of=foo bs=2M count=512 oflag=direct
512+0 records in
512+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 0.504496 s, 2.1 GB/s

$ time fallocate -Z -l 1G bar # -Z is a new option to do actual zero
real 0m0.171s
user 0m0.001s
sys 0m0.003s

b) Create a 10GB zeroed file.
$ dd if=/dev/zero of=foo bs=2M count=5120 oflag=direct
5120+0 records in
5120+0 records out
10737418240 bytes (11 GB, 10 GiB) copied, 5.04009 s, 2.1 GB/s

$ time fallocate -Z -l 10G bar
real 0m1.724s
user 0m0.000s
sys 0m0.024s

2. Run fio overwrite and fallocate with force zero range simultaneously,
fallocate has little impact on write bandwidth and only slightly
affects write latency.

a) Test bandwidth costs.
$ fio -directory=/test -direct=1 -iodepth=10 -fsync=0 -rw=write \
-numjobs=10 -bs=2M -ioengine=libaio -size=20G -runtime=20 \
-fallocate=none -overwrite=1 -group_reportin -name=bw_test

Without background zero range:
bw (MiB/s): min= 2068, max= 2280, per=100.00%, avg=2186.40

With background zero range:
bw (MiB/s): min= 2056, max= 2308, per=100.00%, avg=2186.20

b) Test write latency costs.
$ fio -filename=/test/foo -direct=1 -iodepth=1 -fsync=0 -rw=write \
-numjobs=1 -bs=4k -ioengine=psync -size=5G -runtime=20 \
-fallocate=none -overwrite=1 -group_reportin -name=lat_test

Without background zero range:
lat (nsec): min=9269, max=71635, avg=9840.65

With a background zero range:
lat (usec): min=9, max=982, avg=11.03

3. Compare overwriting in a pre-allocated unwritten file and a written
file in O_DSYNC mode. Write to a file with written extents is much
faster.

# First mkfs and create a test file according to below three cases,
# and then run fio.

$ fio -filename=/test/foo -direct=1 -iodepth=1 -fdatasync=1 \
-rw=write -numjobs=1 -bs=4k -ioengine=psync -size=5G \
-runtime=20 -fallocate=none -group_reportin -name=test

unwritten file: IOPS=20.1k, BW=78.7MiB/s
unwritten file + fast_commit: IOPS=42.9k, BW=167MiB/s
written file: IOPS=98.8k, BW=386MiB/s

Thanks,
Yi.

---

[1] https://nvmexpress.org/specifications/
NVM Command Set Specification, section 3.2.8
[2] https://lore.kernel.org/linux-ext4/20241220011637.1157197-1-yi.zhang@huaweicloud.com/
[3] Here is a simple support of xfs_io and fallocate tool in util-linux.
Feel free to give it a try.

1. util-linux
diff --git a/sys-utils/fallocate.c b/sys-utils/fallocate.c
index ac7c687f2..a2bfa8d39 100644
--- a/sys-utils/fallocate.c
+++ b/sys-utils/fallocate.c
@@ -66,6 +66,10 @@
# define FALLOC_FL_INSERT_RANGE 0x20
#endif

+#ifndef FALLOC_FL_WRITE_ZEROES
+# define FALLOC_FL_WRITE_ZEROES 0x80
+#endif
+
#include "nls.h"
#include "strutils.h"
#include "c.h"
@@ -95,6 +99,7 @@ static void __attribute__((__noreturn__)) usage(void)
fputs(_(" -o, --offset <num> offset for range operations, in bytes\n"), out);
fputs(_(" -p, --punch-hole replace a range with a hole (implies -n)\n"), out);
fputs(_(" -z, --zero-range zero and ensure allocation of a range\n"), out);
+ fputs(_(" -w, --write-zeroes write zeroes and ensure allocation of a range\n"), out);
#ifdef HAVE_POSIX_FALLOCATE
fputs(_(" -x, --posix use posix_fallocate(3) instead of fallocate(2)\n"), out);
#endif
@@ -305,6 +310,7 @@ int main(int argc, char **argv)
{ "dig-holes", no_argument, NULL, 'd' },
{ "insert-range", no_argument, NULL, 'i' },
{ "zero-range", no_argument, NULL, 'z' },
+ { "write-zeroes", no_argument, NULL, 'w' },
{ "offset", required_argument, NULL, 'o' },
{ "length", required_argument, NULL, 'l' },
{ "posix", no_argument, NULL, 'x' },
@@ -313,9 +319,10 @@ int main(int argc, char **argv)
};

static const ul_excl_t excl[] = { /* rows and cols in ASCII order */
- { 'c', 'd', 'p', 'z' },
+ { 'c', 'd', 'p', 'z', 'w' },
{ 'c', 'n' },
- { 'x', 'c', 'd', 'i', 'n', 'p', 'z'},
+ { 'w', 'n' },
+ { 'x', 'c', 'd', 'i', 'n', 'p', 'z', 'w'},
{ 0 }
};
int excl_st[ARRAY_SIZE(excl)] = UL_EXCL_STATUS_INIT;
@@ -325,7 +332,7 @@ int main(int argc, char **argv)
textdomain(PACKAGE);
close_stdout_atexit();

- while ((c = getopt_long(argc, argv, "hvVncpdizxl:o:", longopts, NULL))
+ while ((c = getopt_long(argc, argv, "hvVncpdizwxl:o:", longopts, NULL))
!= -1) {

err_exclusive_options(c, longopts, excl, excl_st);
@@ -355,6 +362,9 @@ int main(int argc, char **argv)
case 'z':
mode |= FALLOC_FL_ZERO_RANGE;
break;
+ case 'w':
+ mode |= FALLOC_FL_WRITE_ZEROES;
+ break;
case 'x':
#ifdef HAVE_POSIX_FALLOCATE
posix = 1;

2. xfs_io
diff --git a/io/prealloc.c b/io/prealloc.c
index 8e968c9f..96daf1a1 100644
--- a/io/prealloc.c
+++ b/io/prealloc.c
@@ -30,6 +30,10 @@
#define FALLOC_FL_UNSHARE_RANGE 0x40
#endif

+#ifndef FALLOC_FL_WRITE_ZEROES
+#define FALLOC_FL_WRITE_ZEROES 0x80
+#endif
+
static cmdinfo_t allocsp_cmd;
static cmdinfo_t freesp_cmd;
static cmdinfo_t resvsp_cmd;
@@ -377,6 +381,28 @@ funshare_f(
return 0;
}

+static int
+fwrite_zeroes_f(
+ int argc,
+ char **argv)
+{
+ xfs_flock64_t segment;
+ int mode = FALLOC_FL_WRITE_ZEROES;
+
+ if (!offset_length(argv[1], argv[2], &segment)) {
+ exitcode = 1;
+ return 0;
+ }
+
+ if (fallocate(file->fd, mode,
+ segment.l_start, segment.l_len)) {
+ perror("fallocate");
+ exitcode = 1;
+ return 0;
+ }
+ return 0;
+}
+
void
prealloc_init(void)
{
@@ -489,4 +515,14 @@ prealloc_init(void)
funshare_cmd.oneline =
_("unshares shared blocks within the range");
add_command(&funshare_cmd);
+
+ funshare_cmd.name = "fwrite_zeroes";
+ funshare_cmd.cfunc = fwrite_zeroes_f;
+ funshare_cmd.argmin = 2;
+ funshare_cmd.argmax = 2;
+ funshare_cmd.flags = CMD_NOMAP_OK | CMD_FOREIGN_OK;
+ funshare_cmd.args = _("off len");
+ funshare_cmd.oneline =
+ _("zeroes space and eliminates holes by allocating and writing zeroes");
+ add_command(&funshare_cmd);
}

Zhang Yi (8):
block: introduce BLK_FEAT_WRITE_ZEROES_UNMAP to queue limits features
nvme: set BLK_FEAT_WRITE_ZEROES_UNMAP if device supports DEAC bit
scsi: sd: set BLK_FEAT_WRITE_ZEROES_UNMAP if device supports unmap
zeroing mode
dm: add BLK_FEAT_WRITE_ZEROES_UNMAP support
fs: introduce FALLOC_FL_WRITE_ZEROES to fallocate
block: add FALLOC_FL_WRITE_ZEROES support
block: factor out common part in blkdev_fallocate()
ext4: add FALLOC_FL_WRITE_ZEROES support

Documentation/ABI/stable/sysfs-block | 14 +++++++
block/blk-settings.c | 6 +++
block/blk-sysfs.c | 3 ++
block/fops.c | 37 +++++++++--------
drivers/md/dm-table.c | 3 +-
drivers/nvme/host/core.c | 21 +++++-----
drivers/scsi/sd.c | 5 +++
fs/ext4/extents.c | 59 ++++++++++++++++++++++------
fs/open.c | 1 +
include/linux/blkdev.h | 3 ++
include/linux/falloc.h | 3 +-
include/trace/events/ext4.h | 3 +-
include/uapi/linux/falloc.h | 18 +++++++++
13 files changed, 134 insertions(+), 42 deletions(-)

--
2.39.2