Misc.APDTimeout Archives

I just finished writing the vMSC 6.0 Best Practices paper which is about to be released when a question came in. The question was around the APD scenario and whether the response to an APD should be set to aggressive or conservative. Its a good question and my instinct immediately say: conservative… But should it be configured to that in all cases? If so, why on earth do we even have an aggressive method? That got me thinking. (By the way, make sure to read this article by Matt Meyer on VMCP on the vSphere blog, good post!) But before I spill the beans, what is aggressive/conservative in this case, and what is this feature again?

VM Component Protection (VMCP) is new in 6.0 and it allows vSphere to respond to a scenario where the host has lost access to a storage device. (Both PDL and APD.) In previous releases, vSphere was already capable of responding to PDL scenarios but the settings weren’t really exposed in the UI and that has been done with 6.0 and the APD response has also been added at the same time. Great feature if you ask me, especially in stretched environments as it will help during certain failure scenarios. [Read more…] about Conservative vs Aggressive for VMCP APD response

I’ve written about Permanent Device Loss multiple times but another scenario that some of you might have encountered is All Paths Down. All Paths Down already describes the scenario, but an example would be when for whatever reason the network between the host and the array fails. This would be result in an APD condition, meaning that the LUNs are unreachable due to the fact that all paths to the LUN are gone.

Some of you who have been in this scenario probably also have seen hosts being disconnected. In some cases, I’ve seen this happening at one point, a host might even freeze up. This would typically happen when a lot of I/O was sent to the datastore. This is of course something that everyone would want to avoid and hence a new advanced setting has been introduced, a new mechanism to handle APD conditions.

This brand new setting is called Misc.APDHandlingEnable. It can be set to 0 or 1. A value of zero means that ESXi will stick to the “old” method which is to always retry failed I/O’s. A value of 1 enables the new behavior. The behavior will allow ESXi to “fast-fail” I/Os. This will happen after 140 seconds by default. Fast-failing I/Os is what will prevent the host to be disconnected or frozen up. This is configurable though through Misc.APDTimeout. Note you can set a filter in the Web Client to find the right advanced setting as shown in the screenshot below. Note that the minimum value for Misc.APDTimeout is 20 seconds.

Cormac Hogan has a great article about APD with a lot more technical details, make sure to read it.