On 2014-07-03 18:20, Elliott, Robert (Server Storage) wrote:
Doug Gilbert noticed while running fio to scsi_debug devices
with the scsi-mq.2 tree that it is generating frequent ioctl
calls (e.g., 35 times per second on my system):
[ 1324.777541] sd 5:0:0:0: scsi_debug_ioctl: BLKFLSBUF [0x1261]
[ 1324.782543] sd 5:0:4:0: scsi_debug_ioctl: BLKFLSBUF [0x1261]
[ 1324.800988] sd 5:0:4:0: scsi_debug_ioctl: BLKFLSBUF [0x1261]
[ 1324.802529] sd 5:0:2:0: scsi_debug_ioctl: BLKFLSBUF [0x1261]
[ 1324.805116] sd 5:0:5:0: scsi_debug_ioctl: BLKFLSBUF [0x1261]
[ 1324.811526] sd 5:0:1:0: scsi_debug_ioctl: BLKFLSBUF [0x1261]
[ 1324.813527] sd 5:0:2:0: scsi_debug_ioctl: BLKFLSBUF [0x1261]
They come from fio's invalidate option.
Although the man page says:
invalidate=bool
Invalidate buffer-cache for the file prior
to starting I/O. Default: true.
the invalidations are happen on many io_units, not just once
at startup. Setting invalidate=0 makes them go away. However,
the root cause is a bunch of closes.
This is the call chain (fio-2.1.10-22-g5eba):
do_io
get_io_u /* Return an io_u to be processed. Gets a buflen and offset, sets direction */
set_io_u_file
get_next_file
__get_next_file
get_next_file_rand
td_io_open_file
file_invalidate_cache
__file_invalidate_cache
blockdev_invalidate_cache
return ioctl(f->fd, BLKFLSBUF);
which causes the linux block layer to run fsync_bdev and
invalidate_bdev.
The device/file keeps getting closed by backend.c thread_main
in this loop:
while (keep_running(td)) {
...
if (clear_state)
clear_io_state(td);
...
if (...
else
verify_bytes = do_io(td);
clear_state = 1;
...
}
via this call chain:
clear_io_state
close_files
td_io_close_file
so it keeps having to reopen the file, and asks for a flush
each time.
Are those clear_io_state/close_files calls really intended?
fio script:
[global]
direct=1
ioengine=libaio
norandommap
randrepeat=0
bs=4096
iodepth=96
numjobs=6
runtime=216000
time_based=1
group_reporting
thread
gtod_reduce=1
iodepth_batch=16
iodepth_batch_complete=16
cpus_allowed=0-5
cpus_allowed_policy=split
rw=randread
[4_KiB_RR_drive_ah]
filename=/dev/sdah
How big are the devices? Should only open/close once per reading it, but
if it's scsi_debug and they are small, then that might explain it. If
that's not the case, it's definitely a but and we'll need to look into it.
--
Jens Axboe
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