When volumes are configured to use Most Recently Used, an imbalance of 100% is usually observed (ESXi tends to select paths that lead to the same front end port for all devices). One method that is used to confirm this is to check the I/O balance from each host across both controllers. Because of this, we want to ensure that all hosts are actively using both controllers prior to upgrade. Due to the reboots, twice during the process half of the FlashArray front-end ports go away. When Purity is upgraded on a FlashArray, the following process is observed (at a high level): upgrade Purity on one controller, reboot it, wait for it to come back up, upgrade Purity on the other controller, reboot it and you’re done. This greatly reduces the impact of a physical failure in the storage environment and provides greater performance resiliency and reliability. When this value is set to the minimum of 1, path failover generally decreases to sub-ten seconds. When the I/O Operations Limit is set to the default of 1,000 path failover time can sometimes be in the 10s of seconds which can lead to noticeable disruption in performance during this failure. This failure does not always happen immediately. During a physical failure of the storage environment (loss of a HBA, switch, cable, port, controller) ESXi, after a certain period of time, will fail any logical path that relies on that failed physical hardware and will discontinue attempting to use it for a given volume. It has been noted in testing that ESXi will fail logical paths much more quickly when this value is set to a the minimum of 1. Regardless, changing this value can improve performance in some use cases, especially with iSCSI. While changing this value from 1,000 to 1 can improve performance, it generally will not solve a major performance problem. While this is true in certain cases, the performance impact of changing this value is not usually profound (generally in the single digits of a percentage performance increase). Often the reason cited to change this value is performance. This recommendation is made for a few reasons: This will cause ESXi to change logical paths after every single I/O, instead of 1,000. Pure Storage recommends tuning this value down to the minimum of 1. Once it has issued 1,000 I/Os for that device, down that path, it will switch to a new logical path and so on. In other words, ESXi will choose a logical path, and start issuing all I/Os for that device down that path. By default, when Round Robin is enabled on a device, ESXi will switch to a new logical path every 1,000 I/Os. The I/O Operations Limit (sometimes called the “IOPS” value) dictates how often ESXi switches logical paths for a given device. The Round Robin Path Selection Policy allows for additional tuning of its path-switching behavior in the form of a setting called the I/O Operations Limit.
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