Uploaded November 2025 | Updated September 2026, 2 weeks ago
Command Duration Limits (CDL) is a QoS protocol for SCSI and ATA HDDs that provides the host with a model of traffic classes and command execution policies that enable a drive to optimize execution of consumed commands. The standard has a two dimensional model. CDL defines Duration Limits by which the host can set general priority for classes of commands and it defines Duration Limit Policies to further guide the device on the desired behavior of commands as they approach the limits of the Duration Limits. CDL has been adopted as SCSI and ATA standards, and implemented by drives, the Linux kernel, and the SAS hardware ecosystem. It is also being considered for adoption for NVMe HDDs. In this talk, we will present the CDL behavioral model, and an overview of the standard as implemented in SCSI and ATA, as well as some thoughts on how this model would be adapted to NVMe. We will also present real world results of CDL’s implementation in the field, showing examples of its benefits on command tail latencies, and also the performance trade-offs involved in maximizing IOPs per TB in today’s ever higher capacity HDDs.
Provide the audience with a clear description of the CDL behavior model. Provide the audience with the performance improvements demonstrated by the implementation of CDL in the field Provide the audience with understanding of the state of support for CDL in the SW and HW ecosystem.
Presented by David Landsman, Western Digital and Damien Le Moal, Western Digital
Command Duration Limits (CDL) is a QoS protocol for SCSI and ATA HDDs that provides the host with a model of traffic classes and command execution policies that enable a drive to optimize execution of consumed commands. The standard has a two dimensional model. CDL defines Duration Limits by which the host can set general priority for classes of commands and it defines Duration Limit Policies to further guide the device on the desired behavior of commands as they approach the limits of the Duration Limits. CDL has been adopted as SCSI and ATA standards, and implemented by drives, the Linux kernel, and the SAS hardware ecosystem. It is also being considered for adoption for NVMe HDDs. In this talk, we will present the CDL behavioral model, and an overview of the standard as implemented in SCSI and ATA, as well as some thoughts on how this model would be adapted to NVMe. We will also present real world results of CDL’s implementation in the field, showing examples of its benefits on command tail latencies, and also the performance trade-offs involved in maximizing IOPs per TB in today’s ever higher capacity HDDs.
Provide the audience with a clear description of the CDL behavior model. Provide the audience with the performance improvements demonstrated by the implementation of CDL in the field Provide the audience with understanding of the state of support for CDL in the SW and HW ecosystem.
Presented by David Landsman, Western Digital and Damien Le Moal, Western Digital
