Problem summary: The IOPS is very unstable since I changed the number of jobs from 2 to 4. even I changed it back, the IOPS performance also can't return back. # cat 1.fio [global] rw=randread size=128m [job1] [job2] when I run fio 1.fio, the iops is around 31k. and then I add the following 2 entries: [job3] [job4] The IOPS dropped to around 1k. Even I remove these 2 jobs, the IOPS still be around 1k. Only if I removed all the jobn.n.0 files, and re-run with 2 jobs setting, the IOPS can be 31k again.
# bash blkinfo.sh /dev/sda Vendor : LSI Model : MR9260-8i Nr_request : 128 rotational : 1
It looks like you're testing against a LSI megaraid SAS controller, which presumably has magnetic drives attached. When you add more jobs to your config its going to cause the heads on the drives (you don't say how many you have) to thrash more as they try and interleave requests that are going to land on different portions of the disk. So its not unsurprising that you'll see IOPS drop off.
A lot of how and where the IOPS will drop off is going to depend on the raid config of the drives you have attached to the controller however. Generally speaking 31k IOPS at 128MB I/O's (which will be split into something smaller like 1MB typically) is well beyond what you should expect 8 HDD's to do unless you're getting lots of hits in the DRAM buffer on the raid controller. Enterprise HDD's (even 15k ones) generally can only sustain <= 250 random read IOPS, so even with perfect interleaving on an 8 drive raid-0, 31k seem suspicious, 1k seems perfectly realistic however!
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