In Xsan, the PIO HiPriWr shows you how latent the connection to your LUNs is. If the connection to any of your LUNs is too high then it can cause instability and worse, potential volume integrity issues. If you run into issues with this kind of latency then you should fix it. But if you can’t, then you can deal with it programatically using the Buffer Cache Size. Increasing the buffer will allow for more caching, which will in turn allow for more latent LUNs to have less effect on the overall performance, health and viability of the SAN. Additionally, the iNode Cache should be increased for the same purpose (although more specifically to allow for iNodes to be written if you have latency on your Metadata LUN(s).
These settings are defined in the volume setup wizard but can be updated in the VOLUMENAME.cfg file of your SAN volume, in /Library/Preferences/FileSystems/Xsan/config.