__send_empty_flush() sends empty flush bios to every target in the
dm_table. However, if the num_targets exceeds the number of block
devices in the dm_table's device list, it could lead to multiple
invocations of __send_duplicate_bios() for the same block device.
Typically, a single thread sending numerous empty flush bios to one
block device is redundant, as these bios are likely to be merged by the
flush state machine. In scenarios where num_targets significantly
outweighs the number of block devices, such behavior may result in a
noteworthy decrease in performance.
This is a real-world scenario that we have encountered:
1) Call fallocate(file_fd, 0, 0, SZ_8G)
2) Call ioctl(file_fd, FS_IOC_FIEMAP, fiemap). In situations of severe
file system fragmentation, fiemap->fm_mapped_extents may exceed 1000.
3) Create a dm-linear device based on fiemap->fm_extents
4) Create a snapshot-cow device based on the dm-linear device
Perf diff of fio test:
fio --group_reporting --name=benchmark --filename=/dev/mapper/example \
--ioengine=sync --invalidate=1 --numjobs=16 --rw=randwrite \
--blocksize=4k --size=2G --time_based --runtime=30 --fdatasync=1
Scenario one:
for i in {0..1023}; do
echo $((8000*$i)) 8000 linear /dev/sda2 $((16384*$i))
done | sudo dmsetup create example
Before: bw=857KiB/
After: bw=30.8MiB/s +3580%
Scenario two:
for i in {0..1023}; do
if [[ $i -gt 511 ]]; then
echo $((8000*$i)) 8000 linear /dev/nvme0n1p6 $((16384*$i))
else
echo $((8000*$i)) 8000 linear /dev/sda2 $((16384*$i))
fi
done | sudo dmsetup create example
Before: bw=1470KiB/
After: bw=33.9MiB/s +2261%
Any comments are welcome!
V4:
-- Fix the build warning "no semicolon at end of struct or union."
V3:
-- Focus on targets with num_flush_bios equal to 1 to simplify the code
-- Use t->devices_lock to protect the dm_table's device list
V2:
-- Split into smaller pieces that are easier to review
-- Add flush_pass_around, suggested by Mikulas Patocka
-- Handling different target types separately
Yang Yang (5):
dm: introduce flush_pass_around flag
dm: add __send_empty_flush_bios() helper
dm: support retrieving struct dm_target from struct dm_dev
dm: Avoid sending redundant empty flush bios to the same block device
dm linear: enable flush optimization function
drivers/md/dm-core.h | 3 +++
drivers/md/dm-ioctl.c | 4 ++++
drivers/md/dm-linear.c | 1 +
drivers/md/dm-table.c | 19 +++++++++++++++++++
drivers/md/dm.c | 34 +++++++++++++++++++++++++---------
include/linux/device-mapper.h | 6 ++++++
6 files changed, 58 insertions(+), 9 deletions(-)
--
2.34.1
