Transfer Latency

Dell Compellent’s Storage Manager has many tools for managing, monitoring, and reporting. I will focus on one performance metric which is transfer latency. Of course, monitoring displays the performance of many aspects of the SAN. Transfer latency monitors more than just the SAN. It measures latency from every point, which is the NIC on the host and everything in between connecting to the back end of the SAN.

Tracking the source of the transfer latency can be difficult since there are many places to check. However, somethings can be done to narrow it down. Storage Manager can drill down to many levels such as the disks to monitor latency. If the transfer latency follows directly with write latency, then the problem is most likely not with Compellent. ESXTOP can be used on the host side, but can be difficult to know exactly what is causing the latency.

Compellent has documentation that pointed me in another direction which was network switch configurations. Compellent has best practice documents available for many switches. Link level flow control needed to be enabled on each switch port the Compellent controllers are connected to on a Cisco Nexus 5548UP switch. This change can be made with no down time to VMs. Change one port’s configuration at a time as a precaution. Below is the configuration provided by one of Compellent’s documents. Transfer latency went dramatically down as soon as the ports were reconfigured.

Switch Configuration Guides for Compellent and EqualLogic SANs

switch# configure
switch(config)#interface ethernet 1/1-32
switch(config-if-range)#priority-flow-control mode off
switch(config-if-range)#flowcontrol send off
switch(config-if-range)#flowcontrol receive on
switch(config-if-range)#exit

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