Apple Mac users are generally a happy group, satisfied with their machines and supplier. But many are served by Windows or Unix servers and herein lies a problem, a limitation. Many NAS devices don't support the Apple protocol, relying on Windows CIFS and the Unix NFS ones instead. Apple users are ill-served in this respect.

Apple is creating a server and storage product line up that can cope with demand from groups of high-end Mac video editing professionals and even, say Apple, from Unix and Windows users, even supercomputers. We have reviewed the Mac OS/X Server, Xserve G5 and Xserve RAID products and here we'll discuss Apple's Xsan product.

This was announced recently in slightly eliptical language. It appears to be a way to virtualise Xserve RAID units - the V-word is not mentioned by Apple - and present a single pool of storage either as block-level items or as files through NAS heads. The main components are an Xsan file system, using 64-bit addressing, through which the storage is presented and a metadata controller.

The XServe RAID boxes store the base data. They are 'seen' by Xsan clients through the metadata controller. An Xsan client can be either an Apple Mac OS/X (Apple Unix) server or Mac OS/X workstation and it runs the Xsan software. This contains two components: the metadata controller and the file system client. Thus the metadata controller runs Mac OS/X and Xsan too. Metadata controller failover is included. Either there can be a specific backup metadata controller or Xsan picks another computer (Xsan client) in the system to be the metadata controller if the current one fails.

The metadata controller manages file locking, space allocation and data access authorisation. Xsan clients connect to the metadata controller over Ethernet to obtain the information needed to access the data in the storage pool over FC.

Xsan clients and the metadata controller link to the Xserve RAID storage by Fibre Channel (FC) and an FC switch, at least one, is needed. Xsan is termed a cluster file system and there can be up to 64 Xsan clients sharing an Xsan volume. There can, however, be many Xsan volumes; Apple doesn't say how many.

Brocade, Emulex and QLogic FC switches can be used. Xsan clients link to them via a dual channel 2Gbit/s Apple HBA. These are the basic Xsan lego blocks.

There is a supposition that part of ADIC's StorNext Management Suite software is used. Comparing descriptions of what it does shows common terminology with that used on the Apple Xsan web site. There is more on this below.

Apple presents several example uses of XSan.

In NAS or traditional IT mode a layer of NAS heads is interposed between the storage switch and the client systems. The NAS heads present files to the clients. These heads run Xsan and are Apple XServe boxes. The clients or data consumers access files via ftp, http or file sharing and don't run Xsan.

In the professional video production environment there is provision for a Capture or Ingest station as a special Xsan client. There is no NAS head involvement here and the Xsan clients link direct to the switch by Fibre Channel and run the Xsan software. To guarantee that very large video files can be written to the SAN or served to a video finishing client, Xsan bandwidth can be reserved for the video streaming that's necessary. Files can be stored on the Xsan system with a RAID level - 0, 1 or 5 for example - via set policies.

A large computational cluster uses the NAS head layer again. In a large cluster each NAS head would service 25-50 cluster nodes. The cluster nodes link to the NAS heads by NFS. Apple says this set up is faster than using a straight NFS setup.

Windows and Unix interoperability
A fourth example will be one where Windows and Unix clients need access to Xsan. They can't run the Xsan software. Instead ADIC's StorNext file system is used. The Windows and Unix boxes see the Xsan through it. ADIC announced the interoperability here

According to Thomas Feil, an executive director EMEA for ADIC software, "There is 100 percent compatibility between the Xsan file system and ADIC's StorNext file system. An Apple Xsan metadata controller and a StorNext metadata controller are functionally the same." This means that a Windows or Unix server can reference an Apple metadata controller and a Mac can access a Unix or Windows-based (ADIC StorNext) metadata controller.

In an ADIC StorNext setup, Windows and Unix clients reference a metadata controller, running the StorNext file system, to find out which blocks to fetch off a Fibre Channel-connected storage resource to satisfy a file need. Just as in the Apple Xsan setup, client systems want files. They get the block addresses of these files from the metadata controller. The Apple Xsan and ADC StorNext environments are identical apart from the clients supported.

This is a form of virtualisation but not at the block level. DataCore and FalconStor virtualise disk drives. Participating clients still have their own logical drives which they don't share with other SAN clients. ADIC, and now Apple, virtualise at the next level up, at the file level. All accessing clients share the same file system but get blocks off the storage arrays. In effect, each accessing client is its own NAS head and the SAN can be used to share files. In effect and for all practical purposes, we can say Apple has ported the StorNext file system to Mac OS/X.

The Xsan Admin tool provides volume management, file system configuration, and remote monitoring. There are user quota and LDAP-based access controls as well. RAID Admin is used to create the RAID sets. Backup can be via ADIC Scalar libraries and Xsan can be managed by the StorNext Management Suite. The Admin tool can also be used remotely.

The storage pool can, Apple says, be readily expanded by adding XServe RAID units or Apple drive modules.

With the provision of facilities suited to video production then Xsan should find a home in Video production shops using Mac workstations. The cumulative effect of having OS/X server HW and SW, Xserve RAID storage and Xsan itself could well incline such customers to have an end-to-end Apple system. It is possible that the Apple Xsan/XServe RAID combination could be used by customers with Windows and Unix systems with ADIC's StorNext SW interposed between them. Whether they will be StorNext customers first and buy the Apple storage concept second can't be foreseen.

Any Apple storage progress out of its set of Mac customers will likely be slow. In-house Apple-based shops inside enterprises may provide a base for such expansion. Any such customers who already use the ADIC StorNext product must be viewed as likely to quickly see the virtues of Xsan.

However, compared to SANs in general there are no facilities for flash copies, mirroring or replication which will make Xsan harder to buy for security-conscious customers. What there is in the StorNext management suite is support for two tiers of disk storage and tape. In other words a 3-tier information lifecycle capability is there.

XSan is not expensive, costing £699 in the UK, $999 in the USA, per system. An Xserve, XRAID hardware and a copy of Xsan could be had for around £21,000, $30,000 in the USA. Xsan is currently in beta release with availability expected in the autumn from the Apple Store. Apple says this simple pricing approach provides a cost around half that of equivalent SAN systems from AVID, IBM or SGI.