I surmise that 2.5" drives are less affected by vibration. I'm surprised they don't use a SDCard as the boot drive. I run our ESX servers from them.
Netbooting -> we're actively looking into that as an option. But you still probably need a local swap drive.
The main advantage of PXE-booting would be maintainability (rolling out upgrades by a simple reboot, etc.) but I assume at your scale you have that already figured out in one way or another.
Either way, thank you for all the insights that you keep sharing with the public!
These hands-on blog posts are priceless both in entertainment and education value. :-)
I used to netboot 5500-6000 Linux boxes that crunched collider data from the CMS detector at the LHC (worked at a DOE lab). I learned a lot that year.