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:

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

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

       •   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 provided.
       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.

       •   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.

       •   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).

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

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

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

       •   maxreadcnt - maximum byte count per read

       •   maxwritecnt - maximum byte count per write

       •   segreadcnt - maximum physically disjoint segments processed per read

       •   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)