While experimenting with some IO loads, I noticed that the random offset selection wasn't as random as it should be. I started looking for something complicated thinking maybe it had to do with large devices. But when I took a step back and tried a simpler test, I found it applied to any size. Using a random map masks the problem because it is forced to cover all the offsets. But for IO loads on large devices with multiple jobs, random maps aren't useful. Here's a simple demonstration on Linux. (Linux only because it uses the special /proc/self/fd files. It can work on other OSs by writing to a normal file or pipe and running the rest of the command on that file/pipe.) First, with a random map... $ ./fio --name=test --rw=randread --iodepth=1 --ioengine=null --thread --bs=128 --size=1k --runtime=6 --time_based --write_iolog=/proc/self/fd/1 | grep 'test.1.0 read' | sort | uniq -c 184946 test.1.0 read 0 128 184945 test.1.0 read 128 128 184945 test.1.0 read 256 128 184946 test.1.0 read 384 128 184945 test.1.0 read 512 128 184946 test.1.0 read 640 128 184945 test.1.0 read 768 128 184945 test.1.0 read 896 128 As expected, it is pretty evenly spread. But now without a random map... $ ./fio --name=test --rw=randread --iodepth=1 --ioengine=null --thread --bs=128 --size=1k --runtime=6 --time_based --norandommap --write_iolog=/proc/self/fd/1 | grep 'test.1.0 read' | sort | uniq -c 188409 test.1.0 read 0 128 565224 test.1.0 read 256 128 188408 test.1.0 read 384 128 565224 test.1.0 read 640 128 Even with a random map, this has an impact. The offsets which are favored by the RNG will always be used first, before those unfavored. Notice that the 3 offsets that got an extra IO were all in the set returned in the second test. (Offsets 0, 384 and 640.) This is not a new regression either. I tried using the OS random flag... $ ./fio --name=test --rw=randread --iodepth=1 --ioengine=null --thread --bs=128 --size=1k --runtime=6 --time_based --norandommap --use_os_rand=1 --write_iolog=/proc/self/fd/1 | grep 'test.1.0 read' | sort | uniq -c 558950 test.1.0 read 0 128 372632 test.1.0 read 384 128 558949 test.1.0 read 512 128 And when I tried going to an old 1.x release before the random changes, it was identical to the use_os_rand flag. -- To unsubscribe from this list: send the line "unsubscribe fio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
