Presently shows optimal bs for the virtual mem "device"

[tukoz]

First, thanks for your paper on this; I liked reading it! Actually 'been some time since it was on my TODO list; which happened today --viva summer time :-)

It seems both of your tests are letting the kernel page cache or virtual memory subsystem takes the lead. It's kind of clearer on the "find out optimal input block size", so I'll report here. Apologies for doing so before I could dig in your scripts.

1. On a fast USB stick (ok fast but)

$ ./dd_ibs_test.sh /media/SDX64_2/dd_ibs_testfile                                                                                                                                                                                    
Generating test file...
block size : transfer rate
     512 : 894 MB/s
    1024 : 1 GB/s
    2048 : 4 GB/s
    4096 : 6 GB/s
    8192 : 5 GB/s
   16384 : 0 GB/s
   32768 : 3 GB/s
   65536 : 2 GB/s
  131072 : 3 GB/s
  262144 : 1 GB/s
  524288 : 1 GB/s
 1048576 : 9 GB/s
 2097152 : 9 GB/s
 4194304 : 7 GB/s
 8388608 : 4 GB/s
16777216 : 0 GB/s
33554432 : 3 GB/s
67108864 : 9 GB/s

2. On SSD

$ sh $HOME/.local/scripts/dd_ibs_test.sh ~/dd_ibs_testfile                                                                                                                                                                                    
Generating test file...
block size : transfer rate
     512 : 896 MB/s
    1024 : 7 GB/s
    2048 : 0 GB/s
    4096 : 9 GB/s
    8192 : 5 GB/s
   16384 : 9 GB/s
   32768 : 4 GB/s
   65536 : 6 GB/s
  131072 : 8 GB/s
  262144 : 5 GB/s
  524288 : 9 GB/s
 1048576 : 7 GB/s
 2097152 : 8 GB/s
 4194304 : 7 GB/s
 8388608 : 1 GB/s
16777216 : 8 GB/s
33554432 : 5 GB/s
67108864 : 8 GB/s

Test system: Ivy 3.3 GHz with Arch Linux.
Linux llewellyn 4.7.2-1-ck #1 SMP PREEMPT Sat Aug 20 14:07:40 EDT 2016 x86_64 GNU/Linux

[tdg5]

Hey @tukoz!

I pushed out some updates this morning that update the scripts to clear the kernel cache on every iteration and also force dd to use fsync mode which should make it skip the fs cache and actually write the blocks to disk before declaring itself done. I think these changes should address your concerns. If not, please let me know and I can reopen this issue.

Thanks for the feedback!