Serial ATA
Despite the lower expectations of home users (and hence the failure of SCSI to take off in that market), PATA is showing its age. PATA interfaces send information across different wires simultaneously -- in parallel -- but this information must be organized at the other end. Physical limitations make it difficult to synchronize these parallel signals at high rates of data transfer. Serial ATA (SATA) solves this problem by sending data in sequence-serially-across the same set of wires. Serialization increases speed by eliminating the timing issues involved in parallel data transfer, even though serial interfaces typically contain fewer wires than their parallel counterparts.

The first version of the SATA standard targets the home market, with improvements including:

• increased speed; initially 150 MB/sec, compared to Ultra ATA/100's 100MB/sec (Maxtor introduced a faster, but less widespread Ultra ATA/133 specification).
• thinner, easier-to-handle cables
• reduced power consumption
• a single, auto-configured drive per cable; no jumper settings or master-slave configuration required.

SATA drives also have some of the intelligent features of SCSI drives, such as command queuing. PATA drives process requests as they arrive; thus, a request for data residing on the other side of the disk will get serviced, even if the next chunk of data could be obtained along the way. Command queuing allows multiple commands to be sent to the drive at once; the drive then determines a more efficient path to the data. Advanced features of this sort are unlikely to influence the home customer as much as the smaller cases SATA's power efficiency and simplified cabling will make possible. Computers with minimal space requirements, such as the numerous imitators of Apple's iMac, are extremely popular.

SATA drives from Seagate (Barracuda ATA V), Western Digital, and other manufacturers are already on the market, although they won't be of much use until 2003, when SATA support will be integrated into motherboards. Since SATA is designed to be compatible with existing software, older computers can be fitted with appropriate host bus adapters to use SATA drives-but this isn't likely to be a popular option. For the small server market, Silicon Image and Promise Technology already market 2 and 4-channel SATA adapters featuring basic RAID capabilities such as mirroring and striping. However, to take advantage of some of SATA's new features, such as hot swapping of failed drives, software changes are necessary. Microsoft has released a FAQ stating that they won't support true booting from or hot swapping of SATA drives initially, which may indicate that similar problems are in store for Intel-based Linux users. Nonetheless, recent posts on the Linux kernel development list point to the potential inclusion of SATA support in the 2.5 kernel, still in beta.

The full SATA specification contains a roadmap for the next decade. Versions 2 and 3 of the standard, in 2004 and 2007 respectively, will add management and reliability features and increase speed even further, putting SATA into a more competitive position as a storage option for the smaller server market. Unfortunately, since SATA initially targets the low-end market, support from commercial Unix vendors, who use SCSI or Fiber Channel almost exclusively, is almost nonexistent.

Infiniband and PCI-X
One of the more interesting SATA statistics is that its top speed of 150 MB/sec exceeds the speed of the well-known Peripheral Component Interconnect (PCI) bus, 133 MB/sec. This doesn't mean that it's possible to saturate a PCI bus with a single hard drive except in theory, but it does stress the need for speedier input/output interconnects. Two standards are competing in this field: Infiniband and PCI Express. Intel recently announced that it would pursue PCI Express instead of Infiniband in its forthcoming chipsets. Shortly thereafter, Microsoft also withdrew Infiniband support from its .NET Server product line. A brief look at the respective specifications of PCI Express and Infiniband reveals the reasons.

PCI Express is the new name for the 3rd Generation I/O (3GIO) standard. It is an evolutionary step, like SATA: a serialized enhancement of an existing parallel standard. Also like SATA, it is backwards compatible: older PCI cards will work in PCI Express slots. As with today's PCI interfaces, PCI Express will be used to connect processors inside the server-for instance, to connect the main CPU to a graphics card. PCI Express will operate at 2.5 GB/sec initially. However, as with PCI, PCI Express is specifically designed for connecting components across short distances-20 inches between components.

Infiniband, on the other hand, is not an evolutionary step from previously existing technology in the manner of SATA or PCI-X; it is an entirely new I/O interface intended to connect servers, storage, processors, or other peripherals. Using a switched interconnect technology, Infiniband shares the I/O subsystem between multiple clients (processors, etc.), allowing them to make I/O requests simultaneously. Infiniband networks can use copper or fiber optic cabling, and support distances up to 10 km with the latter.

Given its relatively long distances, scalable switched architecture, and speeds of up to 30GB/sec, Infiniband is a promising technology for connecting clusters or blade servers -- situations where inter-processor communication (IPC) must be speedy. A recent Microsoft presentation (PowerPoint required) aptly identifies Infiniband as an "inside-the-rack" technology, rather than an inside-the-server one. While Infiniband is a potential competitor for Fibre Channel and other storage network technologies, given time to match Fibre Channel's momentum, it's understandable that Microsoft doesn't consider it their first priority right now. Increasing the speed of the PCI bus in the small server market, where they have a stronger foothold, is more likely to help their sales.

Of the two protocols, PCI Express is more likely to affect Linux users first, because of the combined support of Intel and Microsoft. Infiniband, like Fibre Channel in its early days, will gain a foothold in the large UNIX server market, where shared, quick access to storage and processing is critical.

Convergence
Evolutionary protocols such as SATA and PCI Express aren't intended to shake up the market, but the more revolutionary standards have that potential. New I/O and storage networking protocols such as Infiniband and iSCSI blur the old local-remote distinctions; processors, network interfaces, and storage no longer need to reside in the same box. Confusion arises because some protocols can serve several purposes. Infiniband, for instance, can be used for interprocessor communication in a single machine, between servers in a cluster, or as a SAN technology-so its market is unclear.

It's already common to find separate racks for servers and storage in modern datacenters; Infiniband and its kin will continue this trend until processors, storage, and networking are almost independent of physical location. Today's server, with its integrated Ethernet and disk drive, will be dismembered, and its components turned into shared resource pools. Not all computing can be performed this way, of course. Random-access memory (RAM) and processors will always be bedfellows, because the time it takes electrons to travel the few inches between the two has a measurable impact on performance. In most other respects, though, the need for proximity of components that resulted in the "small gray box" design is being eliminated.

As to what you ought to buy: Today's computer industry is so large that some vendors are opting not to choose between competing protocols, instead supporting them all. On the storage networking front, Cisco's new "storage routers" support iSCSI, Fibre Channel and FCIP. InfiniCon, a new startup, is developing products that can bridge InfiniBand and Fibre Channel fabrics. It's guaranteed that a path between what you have now and whatever you buy in the future will exist. This type of confusion is good.