Mac: Slowing Down Your Disk Speed by 60x
Use macOS's built-in tool `dmc` to simulate slow disk performance for testing. It offers various profiles to adjust speed easily.
Sometimes in the development process, we need to 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.
Using dmc
macOS comes with dmc, which we can explore using dmc -h.
➜ /tmp dmc -h
usage: dmc
# commands:
start (profile_name|profile_index [-boot])
stop
status [-json]
show profile_name|profile_index
list
select (profile_name|profile_index)
configure [ ]
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 simply 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.
