Mac: Slowing Down Your Disk Speed by 60x

Sometimes in the development process, we must simulate slow disk conditions to verify if our code can still function on low-performance machines. Typically, we would use cgroup or Docker for this purpose, but it can be cumbersome on a MacBook. However, there's a built-in tool on macOS that can help us achieve this: dmc.

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Using dmc

macOS comes with dmc, which we can explore using dmc -h.

➜  /tmp dmc -h 
 usage: dmc <commands...> 
 # commands: 
   start <mount> (profile_name|profile_index [-boot]) 
   stop <mount> 
   status <mount> [-json] 
   show profile_name|profile_index 
   list 
   select <mount> (profile_name|profile_index) 
   configure <mount> <type> <access_time> <read_throughput> <write_throughput> [<ioqueue_depth> <maxreadcnt> <maxwritecnt> <segreadcnt> <segwritecnt>] 
   help | -h

It offers various disk profiles to choose from:

➜  /tmp dmc list 
   0: Faulty 5400 HDD 
   1: 5400 HDD 
   2: 7200 HDD 
   3: Slow SSD 
   4: SATA II SSD 
   5: SATA III SSD 
   6: PCIe 2 SSD 
   7: PCIe 3 SSD

Each profile corresponds to different speed modes, such as:

➜  /tmp dmc show 0 
 Profile: Faulty 5400 HDD 
  Type: HDD 
  Access time: 52222 us 
  Read throughput: 50 MB/s 
  Write throughput: 50 MB/s 
  I/O Queue Depth: 16 
  Max Read Bytes: 16777216 
  Max Write Bytes: 16777216 
  Max Read Segments: 128 
  Max Write Segments: 128 
 ➜  /tmp dmc show 7 
 Profile: PCIe 3 SSD 
  Type: SSD 
  Access time: 3 us 
  Read throughput: 3072 MB/s 
  Write throughput: 2560 MB/s 
  I/O Queue Depth: 256 
  Max Read Bytes: 67108864 
  Max Write Bytes: 67108864 
  Max Read Segments: 256 
  Max Write Segments: 256

Using it is straightforward. Suppose our disk is mounted on /tmp/data, and we want to set it to level 0. We execute:

sudo dmc start /tmp/data 0

Then, to verify the status:

➜  /tmp dmc status /tmp/data 
 Disk Mount Conditioner: ON 
 Profile: Custom 
  Type: HDD 
  Access time: 52222 us 
  Read throughput: 50 MB/s 
  Write throughput: 50 MB/s 
  I/O Queue Depth: 16 
  Max Read Bytes: 1048576 
  Max Write Bytes: 1048576 
  Max Read Segments: 128 
  Max Write Segments: 128

Verification

We can validate using the fio tool:

➜  /tmp fio --filename=./data/test1 -direct=1 --rw=write --ioengine=posixaio --bs=1m --iodepth=32 --size=1G --numjobs=1 --runtime=60 --time_base=1 --group_reporting --name=test-seq-read --log_avg_msec=1000

Writing a 1GB file sequentially yields a speed of only 95.4MB/s, with IOPS at 91:

Run status group 0 (all jobs): 
   WRITE: bw=91.0MiB/s (95.4MB/s), 91.0MiB/s-91.0MiB/s (95.4MB/s-95.4MB/s), io=5468MiB (5734MB), run=60073-60073msec

Now, with dmc turned off and the same command:

/tmp sudo dmc stop /tmp/data 
  /tmp fio --filename=./data/test1 -direct=1 --rw=write --ioengine=posixaio --bs=1m --iodepth=32 --size=1G --numjobs=1 --runtime=60 --time_base=1 --group_reporting --name=test-seq-read --log_avg_msec=1000

The test results in 3211MB/s and IOPS of 3061, which represents the true speed of the disk:

WRITE: bw=3062MiB/s (3211MB/s), 3062MiB/s-3062MiB/s (3211MB/s-3211MB/s), io=179GiB (193GB), run=60006-60006msec

In conclusion, dmc is quite handy in testing scenarios. I hadn't known about this software before; if you find it useful, give it a try.