IBM's iSeries mid-range servers just keep on getting refreshed. Unix on the iSeries? Certainly. You want Java on the iSeries? Of course. Is Windows needed on the iSeries? We'll attach a processor. Database? DB2. Messaging? Domino? Virtual servers? Naturally; in OS/400 and Unix of course, oh and Linux too. Storage area networks? No problem. NAS? ermm, well, actually...

Actually, the iSeries is a very clever and long-lived design that started out as a minicomputer with a difference. It has a concept built into the operating system of a single level store. As far as applications are concerned data is stored in memory and memory covers both RAM and disk. It is all one address space as far as applications are concerned.

This is disk virtualisation carried out in such a way that an application's virtual memory includes disk. In Windows, and in other O/S', virtual memory is not apparent to applications. They have non-memory-resident parts of their code written out by the O/S to a swap file on a system disk. This swap file contains bits of several applications and the O/S swaps them in and out of RAM as needed. It's invisible to applications which think of RAM and disk as two logically seprate storage spaces.

This virtualisation makes it easy for the iSeries system to add resources to applications that need them. In fact OS/400, AIX and Linux can run simultaneously on the latest iSeries system, with a hypervisor supervising them in their separate partitions. Each of them can have their own multiple partitions with processor, memory and other resources doled out to them by the hypervisor software. It is a sophisticated memory and disk and server virtualisation infrastructure.

According to Nigel Adams, iSeries product manager for IBM's EMEA north region, the iSeries O/S, virtualises disk so much that the concept of files disappears for applications. OS/400 application software doesn't have to worry about writing files to disk. It just opens files, uses their contents, amends their contents and then closes them. If a database, such as DB2 is involved then it's opened, records are processed, and it's closed.

It does mean that porting applications from OS/400 to Windows or vice versa means they have to have recoding to add or remove file-based operations. An exception is Java-based applications as they are intrinsically portable.

OS/400 knows about RAM and disks and can mirror disks, take advantage of RAID facilities, and back up disks. But the applications don't know the difference. In fact they don't really get concerned with files as they refer to objects. An object is a piece of data; a picture; a document; an executable image; a database; or a script of some sort. The operating system knows what reach object is and what it can do and what can be done with it. There's no possibility of a data object masquerading as an executable image.

We'll make a distinction here between operating systems executing on the iSeries' main processors, the Power4 and now Power5 IBM chips, and Windows executing on an internally-attached xSeries Intel processor or externally-attached xSeries system with up to eight Intel CPUs. These run Windows and the applications running in Windows see files as per normal.

OS/400 applications do not see files; they see objects. In this environment SANs can be readily adopted. A SAN serves blocks and not files. It is an extension of direct-attached storage as far as server O/S and applications are concerned. OS/400 'merely' adds the SAN storage it has been given to the single pool of storage it already maintains.

A network-attached storage system serves files to attached servers. An iSeries server doesn't present files to its applications. They, the applications running in an OS/400 environment, simply cannot see files. The AIX and Linux environments on an iSeries, with storage, meaning memory and disk, presented to them by the hypervisor, also cannot see files.

However, again according to Nigel Adams, Windows running on an attached xSeries processor could use NAS, as it provides both RAM and file-based storage to applications.

To an iSeries application NAS is invisible. Storage consolidation iSeries-style, is at the block level. iSeries applications and iSeries servers cannot share files because they don't 'see' files; they 'see'objects and 'network-attached object stores', if such things existed, would equate to SANs.

This does make iSeries servers different. If you have used NAS to consolidate storage between different servers then any iSeries boxes you might have cannot be brought into that storage consolidation fold. The only way to do it would be to have a SAN with a NAS head. The NAS head would serve files to the non-iSeries servers while the iSeries machines would access the SAN directly.

Any Windows systems running on the iSeries attached XSeries processors could, of course, use the NAS head.