Merge patch series "fallocate: introduce FALLOC_FL_WRITE_ZEROES flag"

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

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. However, please note that this may be a
best-effort optimization rather than a mandatory requirement, some
devices may partially fall back to writing physical zeroes due to
factors such as receiving unaligned commands.

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 if the attached disk supports
   this command.

2. Introduce a queue limit flag, BLK_FLAG_WRITE_ZEROES_UNMAP_DISABLED,
   along with a corresponding sysfs entry. Users can query the support
   status of the unmap write zeroes operation and disable this operation
   if the write zeroes operation is very slow.

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

3. Introduce a new flag, FALLOC_FL_WRITE_ZEROES, into the fallocate.
   Filesystems that support this operation should allocate written
   extents and issue zeroes to the specified range of the device. For
   local block device filesystems, this operation should depend on the
   write_zeroes_unmap operaion of the underlying block device. It should
   return -EOPNOTSUPP if the device doesn't enable unmap write zeroes
   operaion.

This series implements the BLK_FEAT_WRITE_ZEROES_UNMAP feature and
BLK_FLAG_WRITE_ZEROES_UNMAP_DISABLED flag for SCSI, NVMe and
device-mapper drivers, and add the FALLOC_FL_WRITE_ZEROES and
STATX_ATTR_WRITE_ZEROES_UNMAP support for ext4 and raw bdev devices.
Any comments are welcome.

I've 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 unmap write zeroes, the later one is
   significantly faster than 'dd'.

   Create a 1GB and 10GB zeroed file.
    $dd if=/dev/zero of=foo bs=2M count=$count oflag=direct
    $time fallocate -w -l $size bar

    #1G
    dd:                     0.5s
    FALLOC_FL_WRITE_ZEROES: 0.17s

    #10G
    dd:                     5.0s
    FALLOC_FL_WRITE_ZEROES: 1.7s

2. Run fio overwrite and fallocate with unmap write zeroes
   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

* patches from https://lore.kernel.org/20250619111806.3546162-1-yi.zhang@huaweicloud.com:
  ext4: add FALLOC_FL_WRITE_ZEROES support
  block: add FALLOC_FL_WRITE_ZEROES support
  block: factor out common part in blkdev_fallocate()
  fs: introduce FALLOC_FL_WRITE_ZEROES to fallocate
  dm: clear unmap write zeroes limits when disabling write zeroes
  scsi: sd: set max_hw_wzeroes_unmap_sectors if device supports SD_ZERO_*_UNMAP
  nvmet: set WZDS and DRB if device enables unmap write zeroes operation
  nvme: set max_hw_wzeroes_unmap_sectors if device supports DEAC bit
  block: introduce max_{hw|user}_wzeroes_unmap_sectors to queue limits

Link: https://lore.kernel.org/20250619111806.3546162-1-yi.zhang@huaweicloud.com
Signed-off-by: Christian Brauner <brauner@kernel.org>