mojave man page for dmc

DMC(1)																	    DMC(1)

NAME
       dmc - controls the Disk Mount Conditioner

SYNOPSIS
       dmc start mount [profile-name|profile-index [-boot]]
       dmc stop mount
       dmc status mount [-json]
       dmc show profile-name|profile-index
       dmc list
       dmc select mount profile-name|profile-index
       dmc configure mount type access-time read-throughput write-throughput [ioqueue-depth maxreadcnt maxwritecnt segreadcnt segwritecnt]
       dmc help | -h

DESCRIPTION
       dmc(1)  configures  the Disk Mount Conditioner. 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. It can  also  cause  the  conditioned
       mount point to advertise itself as a different device type, e.g. the disk type of an SSD could be set to an HDD. This behavior consequently
       changes various parameters such as read-ahead settings, disk I/O throttling, etc., which normally have different behavior depending on  the
       underlying device type.

COMMANDS
       Common command parameters:

       o   mount - the mount point to be used in the command

       o   profile-name - the name of a profile as shown in dmc list

       o   profile-index - the index of a profile as shown in dmc list

       dmc start mount [profile-name|profile-index [-boot]]
	       Start the Disk Mount Conditioner on the given mount point with the current settings (from dmc status) or the given profile, if pro-
       vided. Optionally configure the profile to remain enabled across reboots, if -boot is supplied.

       dmc stop mount
	   Disable the Disk Mount Conditioner on the given mount point. Also disables any settings that persist across reboot via the  -boot  flag
       provided to dmc start, if any.

       dmc status mount [-json]
	   Display the current settings (including on/off state), optionally as JSON

       dmc show profile-name|profile-index
	   Display the settings of the given profile

       dmc list
	   Display all profile names and indices

       dmc select mount profile-name|profile-index
	   Choose a different profile for the given mount point without enabling or disabling the Disk Mount Conditioner

       dmc configure mount type access-time read-throughput write-throughput [ioqueue-depth maxreadcnt maxwritecnt segreadcnt segwritecnt]
	   Select custom parameters for the given mount point rather than using the settings provided by a default profile.

	   See dmc list for example parameter settings for various disk presets.

       o   type  -  'SSD' or 'HDD'. The type determines how various system behaviors like disk I/O throttling and read-ahead algorithms affect the
	   issued I/O. Additionally, choosing 'HDD' will attempt to simulate seek times, including drive spin-up from idle.

       o   access-time - latency in microseconds for a single I/O. For SSD types this latency is applied exactly as specified to all I/O. For  HDD
	   types, the latency scales based on a simulated seek time (thus making the access-time the maximum latency or seek penalty).

       o   read-throughput - integer specifying megabytes-per-second maximum throughput for disk reads

       o   write-throughput - integer specifying megabytes-per-second maxmimu throughput for disk writes

       o   ioqueue-depth - maximum number of commands that a device can accept

       o   maxreadcnt - maximum byte count per read

       o   maxwritecnt - maximum byte count per write

       o   segreadcnt - maximum physically disjoint segments processed per read

       o   segwritecnt - maximum physically disjoint segments processed per write

       dmc help | -h
	   Display help text

EXAMPLES
       dmc start / '5400 HDD'

	   Turn on the Disk Mount Conditioner for the boot volume, acting like a 5400 RPM hard drive.

       dmc configure /Volumes/ExtDisk SSD 100 100 50

	       Configure an external disk to use custom parameters to degrade performance as if it were a slow SSD with 100 microsecond latencies,
       100MB/s read throughput, and 50MB/s write throughput.

IMPORTANT
       The Disk Mount Conditioner is not a 'simulator'. It can only degrade (or 'condition') the I/O such that a faster disk device behaves like a
       slower  device,	not  vice-versa.  For  example,  a 5400 RPM hard drive cannot be conditioned to act like a SSD that is capable of a higher
       throughput than the theoretical limitations of the hard disk.

       In addition to running dmc stop, rebooting is also a sufficient way to clear any existing settings and disable Disk  Mount  Conditioner	on
       all mount points (unless started with -boot).

SEE ALSO
       nlc(1)

								   January 2018 							    DMC(1)