Disk Mount Conditioning In macOS

The Disk Mount Conditioner “is a kernel provided service that can degrade the disk I/O being issued to specific mount points, providing the illusion that the I/O is executing on a slower device.” You won’t often find that the system decides to slow throughput to a device very often. But it happens, and equally as useful you can spoof a different type of device, quite helpful when troubleshooting. 

You can use the dmc command to control and view the status of the dmc service, at /usr/bin/dmc. To see how to use dmc, imply run dmc with the help verb:

You can see a list of mounts that can be controlled using the list verb:

  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

Many of the above aren’t available, but if you look at your hard drive, you can check the status of it (or multiples if you have multiple supported controllers in your computer). To check the status simply use the list verb following the integer of your mount:

Once you have the list, you can check the status of each until you find the one you need. To check the status simply use the status verb following the integer of your mount:

If you run a status check against a mount that is not available, you’ll get something similar to the following:

DISK_CONDITIONER_IOC_GET error: No such file or directory

If you run the status check on a supported mount, you’ll get output similar to the following:

Disk Mount Conditioner: OFF

Profile: Custom

 Type: HDD

 Access time: 0 us

 Read throughput: 0 MB/s (unlimited)

 Write throughput: 0 MB/s (unlimited)

 I/O Queue Depth: 0

 Max Read Bytes: 0

 Max Write Bytes: 0

 Max Read Segments: 0

 Max Write Segments: 0

Note that the Profile is listed as Custom. There’s a funky thing with this command, where if you want to see how the profile that is applied to a given device is configured – you use the characters  assigned to the list number:

The output would be similar to the following:

Profile: 5400 HDD

 Type: HDD

 Access time: 26111 us

 Read throughput: 100 MB/s

 Write throughput: 100 MB/s

 I/O Queue Depth: 32

 Max Read Bytes: 33554432

 Max Write Bytes: 33554432

 Max Read Segments: 256

 Max Write Segments: 256

So if you start the Disk Mount Conditioner for that device, you’d apply the profile of another in order to do so. So, let’s say you want to slow the device down with a given set of settings:

Now, let’s say I wanted to spoof a PCIe 3 SSD. First I’d need to stop the conditioning service:

Then let’s run the start again with the mount followed by the profile index to invoke:

And of course, to stop again, simply run the stop for the device again:

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