VMW_PSP_FIXED vs. VMW_PSP_FIXED_AP
Recently a vendor asked me what the difference between VMW_PSP_FIXED and VMW_PSP_FIXED_AP is. Since VMW_PSP_FIXED_AP is not specifically listed on the HCL, the vendor was confused on why the SATP had automatically selected VMW_PSP_FIXED_AP instead of VMW_PSP_FIXED.
VMW_PSP_FIXED_AP was first implemented in ESX/ESXi 4.1 and in 5.x it was merged into the main VMW_PSP_FIXED. The difference here is that VMW_PSP_FIXED_AP has added features for ALUA. VMW_PSP_FIXED_AP will act as VMW_PSP_FIXED when Array Preference is not implemented.
PSPs are Path Selection Policies. All that they do is decide which path to use for I/O given a set of paths. These are implemented to be simple as they have to be used with thousands of different arrays. VMW_PSP_FIXED and VMW_PSP_FIXED_AP use a priority system to determine the path to be used.
VMW_PSP_FIXED_AP will use the following criteria when selecting a path.
- User Preferred Path.
- Array Path Preference. If the array does not implement this, this will be skipped.
- Current path state.
The great thing about this is that the array should not let the host use an active non-optimized path.
The real difference is that VMW_PSP_FIXED does not implement #2 and VMW_PSP_FIXED_AP also implements follow-over. Asymmetric state change is explained here in much more detail.
blog comments powered by Disqus
