dmc(1) dmc(1)
NAME
dmc - controls the Disk Mount Conditioner
SYNOPSIS
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
DESCRIPTION
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.
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]
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-
sets.
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
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 per-
formance 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 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.
SEE ALSO
nlc(1)
April 2017 dmc(1)
Mac OS X 10.12.6 - Generated Sun Oct 29 10:38:44 CDT 2017