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dmc(1)                                                                  dmc(1)


       dmc - controls the Disk Mount Conditioner


       dmc start mount [profile-name|profile-index]
       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
       dmc help | -h


       dmc(1)  configures  the  Disk  Mount Conditioner. The Disk Mount Condi-
       tioner 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 throt-
       tling, etc., which normally have different behavior  depending  on  the
       underlying device type.


       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]
            Start the Disk Mount Conditioner on the given mount point with the
       current settings (from dmc status) or the given profile, if provided.

       dmc stop mount
           Disable the Disk Mount Conditioner on the given mount point

       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
           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 pre-

       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  simu-
           late 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

       dmc help | -h
           Display help text


       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 per-
       formance as if it were a  slow  SSD  with  100  microsecond  latencies,
       100MB/s read throughput, and 50MB/s write throughput.


       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 can-
       not 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.



                                  April 2017                            dmc(1)

Mac OS X 10.12.6 - Generated Sun Oct 29 10:38:44 CDT 2017
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