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Determining the Cause of Unusual Disk Latency
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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)
