Re: lots of closes causing lots of invalidates while running

[Jens Axboe <axboe@kernel.dk>]

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