What are the advantages to disk subsystems?

I'm assuming you are referring to "centralized disk subsystems." When comparing to direct attached storage (DAS), which was the standard for a long time in the open systems world, disk subsystems offer a compelling number of advantages. The first one and most capitalized on by vendors would be better utilization by centralizing disk storage. By only allocating what storage is needed to each system, the remaining disk space can be pooled and used as needed by other systems connected to the same array. Disk space sharing other than via a network protocol such as NFS was not possible with DAS. This often translated in larger-than-needed disks installed in systems with no practical way of allocating the remaining space to other systems.

Some of the other advantages can be listed as follows:

• Storage allocation without the need for an outage
• Dynamic storage expansion
• Multiple host connections especially when implementing a SAN
• Multi-platform support
• LAN free data movements (backups) via a SAN
• Software or hardware driven remote data replication for DR purposes
• Nways clusters sharing the same disk space made possible by multiple host connections to the storage array