[Question]] How to get the expected results by writing to a single file

[Question]] How to get the expected results by writing to a single file

[Nakajima Akira]

Hi.

I am measuring the speed of the cryptographic product LUKS.
In the case of CBC mode (used by LUKS ver1), due to the mechanism of 
CBC, when writing, it is expected that even if there are 2 threads, only 
the same writing speed as 1 thread.

However, in the following fio command, two files are created, and each 
thread writes to each file, so the speed is twice that of a single thread.
# fio -directory=/luks1 -direct=1 -ioengine=libaio -rw=write -numjobs=2

   This is related to https://www.spinics.net/lists/fio/msg09445.html


So I ran the following instead of -directory, but it was nearly twice as 
fast.
# fio -filename=/luks1/testfile -iodepth=1 -iodepth_batch_complete_min=0 
-direct=1 -ioengine=libaio -rw=write -runtime=60 -bs=1m -size=16G -numjobs=x

numjobs   Result
1         BW=255MiB/s
2         BW=455MiB/s


How can I get the expected result? (Result that numjobs=1 and 2 have the 
same speed)

fio was built today's github (fio-3.26).


Thanks.
Nakajima