Hi Jens,
sorry for the following silly question, but maybe you can solve very
quickly a doubt for which I'd spend much more time investigating.
While doing some tests with scsi_debug, I've just seen that (at least)
with direct I/O, the maximum number of pending I/O requests (at least
in the I/O schedulers) is equal, unexpectedly, to the queue depth of
the drive and not to
/sys/block/<dev>/queue/nr_requests
For example, after:
sudo modprobe scsi_debug max_queue=4
and with fio executed as follows:
job: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=libaio, iodepth=20
I get this periodic trace, where four insertions are followed by four
completions, and so on, till the end of the I/O. This trace is taken
with none, but the result is the same with bfq.
fio-20275 [001] d... 7560.655213: 8,48 I R 281088 + 8 [fio]
fio-20275 [001] d... 7560.655288: 8,48 I R 281096 + 8 [fio]
fio-20275 [001] d... 7560.655311: 8,48 I R 281104 + 8 [fio]
fio-20275 [001] d... 7560.655331: 8,48 I R 281112 + 8 [fio]
<idle>-0 [001] d.h. 7560.749868: 8,48 C R 281088 + 8 [0]
<idle>-0 [001] dNh. 7560.749912: 8,48 C R 281096 + 8 [0]
<idle>-0 [001] dNh. 7560.749928: 8,48 C R 281104 + 8 [0]
<idle>-0 [001] dNh. 7560.749934: 8,48 C R 281112 + 8 [0]
fio-20275 [001] d... 7560.750023: 8,48 I R 281120 + 8 [fio]
fio-20275 [001] d... 7560.750196: 8,48 I R 281128 + 8 [fio]
fio-20275 [001] d... 7560.750229: 8,48 I R 281136 + 8 [fio]
fio-20275 [001] d... 7560.750250: 8,48 I R 281144 + 8 [fio]
<idle>-0 [001] d.h. 7560.842510: 8,48 C R 281120 + 8 [0]
<idle>-0 [001] dNh. 7560.842551: 8,48 C R 281128 + 8 [0]
<idle>-0 [001] dNh. 7560.842556: 8,48 C R 281136 + 8 [0]
<idle>-0 [001] dNh. 7560.842562: 8,48 C R 281144 + 8 [0]
Shouldn't the total number of pending requests reach
/sys/block/<dev>/queue/nr_requests ?
The latter is of course equal to 8.
Thanks,
Paolo
