IBM® 3390 disk drives are commonly used on current mainframes.

Conceptually, this is a simple arrangement, as shown in Figure 1.

Figure 1. Initial IBM 3390 disk implementation

The associated control unit (3990) typically has four channels connected to one or more processors (probably with a switch), and the 3390 unit typically has eight or more disk drives. Each disk drive has the characteristics explained earlier. This illustration shows 3990 and 3390 units, and it also represents the concept or architecture of current devices.

The current equivalent device is an IBM 2105 Enterprise Storage Server®, simplistically illustrated in Figure 2.

Figure 2. Current 3390 implementation

The 2105 unit is a very sophisticated device. It emulates a large number of control units and 3390 disk drives. It contains up to 11 TB of disk space, has up to 32 channel interfaces, 16 GB cache, and 284 MB of non-volatile memory (used for write queueing). The Host Adapters appear as control unit interfaces and can connect up to 32 channels (ESCON® or FICON®).

The physical disk drives are commodity SCSI-type units (although a serial interface, known as SSA, is used to provide faster and redundant access to the disks). A number of internal arrangements are possible, but the most common involves many RAID 5 arrays with hot spares. Practically everything in the unit has a spare or fallback unit. The internal processing (to emulate 3990 control units and 3390 disks) is provided by four high-end RISC processors in two processor complexes; each complex can operate the total system. Internal batteries preserve transient data during short power failures. A separate console is used to configure and manage the unit.

The 2105 offers many functions not available in real 3390 units, including FlashCopy®, Extended Remote Copy, Concurrent Copy, Parallel Access Volumes, Multiple Allegiance, a huge cache, and so forth.

A simple 3390 disk drive (with control unit) has different technology from the 2105 just described. However, the basic architectural appearance to software is the same. This allows applications and system software written for 3390 disk drives to use the newer technology with no revisions.

There have been several stages of new technology implementing 3390 disk drives; the 2105 is the most recent of these. The process of implementing an architectural standard (in this case the 3390 disk drive and associated control unit) with newer and different technology while maintaining software compatibility is characteristic of mainframe development. Maintaining application compatibility over long periods of technology change is an important characteristic of mainframes.