The degree of difficulty or ease in configuring and using LUNs that an HBA discovers largely depends on the OS supporting the HBA.
Under AIX and HP-UX, a system administrator only needs to run the AIX cfgmgr or the HP-UX SAM utility to discover new LUNs. Using this utility, it doesn't matter whether or not the LUNs reside on the same storage array ports as other LUNs it already has discovered. However, you need to check with IBM or HP before purchasing just any HBA, as none of the HBA vendors provide drivers for either AIX or HP-UX. Both HP and IBM develop all of their own HBA drivers, so if they don't provide a driver for the HBA you are interested in, you're out of luck.
Sun Microsystems is in a period of transition right now. Under older versions of Sun Solaris or when using older HBAs, the system administrator must set up the corresponding configuration files. This requires the administrator to obtain the storage array port world wide name (WWN) and the list of LUNs that the storage array port will present to the server. However, Sun is in the process of changing this approach: In Solaris 8, the leadville driver eliminates many of the tedious manual configuration tasks though leadville currently only works with the QLogic 2342 HBA. The good news is that most HBA vendors provide drivers for Sun Solaris and these drivers actually offer most--if not more--functionality than what Sun's leadville driver offers.
The degree of support for LUN discovery under Windows varies quite a bit, depending on the version of Windows you're using. Under the Windows NT 4.0 operating system, the system administrator is always forced to do a reboot to discover a new LUN. Complicating the matter, the LUN is presented as a drive letter, so as a result NT has a built-in limitation of 26 LUNs. Windows 2000 addresses these specific limitations of NT 4.0. It offers the ability to add new volumes or span volumes across several disks without taking the server offline using its new dynamic volume management (DVM) capabilities. It also now offers the ability to manage up to 2,500 LUNs, breaking the previous 26 LUN limitation of Windows NT substantially.
As part of ensuring HBA interoperability with its operating system, Microsoft now also offers driver signing for HBA vendors who go through their certification process. Customers will be able to ascertain an HBA vendor's compliance with a specific release of Windows by looking for the appropriate Windows logo on each HBA vendor's technical documentation.