ATA, S-ATA, SCSI, FC-AL, FC-SF, FC-PP

In the world of storage the latest ATA/100 or ATA/133 drive at 160 Gb is fine to hold your movies, MP3s and whatnot, but what if you’re constantly accessing files all over the drive and need alot of speed? Most people would just get a RAID adapter, make it RAID 0 and call it a day. Is this the best solution though? Not by a long shot. I’m going to briefly go over the three other technologies available (or soon to be available) to give you an idea of their performance advantages. I will not discuss iSCSI as right now it is immature and has not found a market niche.

First there was ATA, ran in simple PIO mode in DOS (before it was multitasking, so PIO mode was perfect) on the IBM PC-AT (AT stands for Advanced Technology, ATA for Advanced Technology Attachment, and you see it everwhere including motherboard form factors). Several years later it’s not transmitting at up to 133 Mb/s with full DMA support and is by far the most prevelant IDE interface. However, there are several disadvantages to the ATA bus as it stands. First, not only can only one device use the bus at a time (which is very common) but a device actually holds the bus the entire time a read or write is being services, leading to large performance penalties. The ability to not hold the bus is included with the SCSI disconnect feature. In addition, hot-swap is barely supported if at all. If you should put two drives on the same bus and the master fails, the slave will become unusable as well. This is by a rather nasty issue designed into the system and is why the more serious ATA RAID controllers only allow for master drives to enable hot-swap. Furthermore, people will often end up with tons and tons of wide, flat cables draped around their system to support all of their devices, and while the rounded cables are better for airflow they’re still a large element.

The replacement to ATA called Serial ATA (or S-ATA) is starting to hit the market now. Rather than using wide, flat ribbon cables it uses a thin serial cable with one cable per drive. Transmitting initially at 150 Mb/s, it enabled hot-swap and makes the bus sharing issue a moot point. Soon this will replace standard ATA.

Big brother to S-ATA, however, is SCSI. Ranging in bandwith from 5 Mb/s to 320 Mb/s, SCSI currently supports the fastest drives with spindle speeds of 10,000 and 15,000 RPM far exceeding the more pedestrian 7200, 5400 and 4800 of ATA drives. This allows them to achieve outer-zone transfer rates approaching 80 Mb/s, allowing four drives to completely saturate an Ultra 320 bus. In addition, because of disconect, tagged command queuing and elevator seeking the drives are utilized in the most effecient method possible in a given situation of combined reads writes. In addition, since the bus can handle so much bandwith it is not uncommon for a high-end workstation to have a single SCSI ribbon cable running through the entireity of it. SCSI is used where speed and good capacity are important for disk and CD-ROM drives, but you can hook up many other devices through SCSI as well. Modems, scanners, printers, scientific insturments, et cetera are all available to be attached through the SCSI bus. Since SCSI was designed from the ground up to be a fast, robust interface it is no wonder it’s still with it and shows no signs of slowing down.


Finally comes the biggest of the big boys. Fibre Channel. While many people have never heard of FC, it is the ultimate for external storage and even internal storage where many, many drives are involved. Fibre Channel comes in three variants, all of which use the same adapter card. The first is AL or Arbitrated Loop. This is where the FC devices are daisy chained into a large ring allowing for failure of any one device or link while the rest of the loop remains intact. This is very common as a way of providing redundancy on a small scale. SF or Switched Fabric is where the cards are plugged into switches (much like ethernet switches) and deviced plugged into those switches as wel. Usually the switch is redundant, and this, although more reliable than AL, is much more expensive. The cheapest and the one people would be looking at for their personal use is PP or Point to Point. Basiaclly, you plug the card into your first drive then daisy chain and off you go. FC uses 10,000 and 15,000 RPM drives, often is used in conjunction with a FC to SCSI bridge or FC to SCSI RAID controller, transmits data at 4 Gigabits per second or higher (approx 400 Mb/s), is full duplex, and can have cable lengths up to 100 Km (yes, kilometers), allowing servers in different buildings to share the same storage array or for redundant storage arrays to be created across town, or even allowing a bank of servers to share one storage array on a machine room floor. Oh, and the cables are thin, too. So if you want the ultimate: look no further than FC.

Let me know if there are any questions or comments.