For the majority of the environments, we have found that the defaults work just fine. Since ESXi is able to queue I/O and abstract some of the underlying storage problems away from VMs and applications, often times environments are unscathed by this I/O pause. More often than not it is more of a concern because somebody saw the pause in I/O and is worried something was impacted or is going to be impacted. So don’t panic if you see this pause, simply assess your environment and verify whether or not everything survived. If it did, great! You know these values work for you.
If on the off-chance it didn’t survive, then luckily there are ways to fix this. Below is what we have found to be safe values for environments that needed slightly better failover times:
NoopOutInterval – 5 seconds
NoopOutTimeout – 10 seconds
RecoveryTime – 10 seconds
This would give you a maximum of 25 seconds in I/O pause but have found that this often results in 15-16 seconds of failover time, which has been more than sufficient for customers we have worked with in times past.
Remember that modifying these values requires two things:
1. Restarting the iSCSI sessions and/or a reboot of the ESXi host to take effect.
2. Thorough testing in your own environment to ensure it meets both your needs and doesn’t cause additional unwanted side effects.
While the results above have worked well for us and customers we have worked with in the past, it doesn’t guarantee it will work for you. The most important thing is to ensure that it works for your environment.