I've 5 Debian Linux servers with HP SAN box. Should I boot from SAN?
No, use centralized network storage for shared data or high availability configuration only. Technically you can boot and configure system. However I don't recommend booting from SAN or any other central server until and unless you need diskless nodes:
[a] Use local storage - Always use local storage for /boot and / (root) filesystem
[b] Keep it simply - Booting from SAN volumes is complicated procedure. Most operating systems are not designed for this kind of configuration. You need to modify scripts and booting procedure.
[c] SAN booting support - Your SAN vendor must support platform booting a Linux server. You need to configure HBA and SAN according to vendor specification. You must totally depend upon SAN vendor for drivers and firmware (HBA Bios) to get thing work properly. General principle - don't put all your eggs in one basket err one vendor ;)
[d] Other factors - Proper fiber channel topology must be used. Make sure Multipathing and redundant SAN links are used. The boot disk LUN is dedicated to a single host. etc
As you can see, complications started to increases, hence I don't recommend booting from SAN.