Latest Developer Headlines · Open Source Targets Microsoft Exchange [BusinessWeek] · McDonald's Offers WiFi [Seattle Post Intelligencer ] · Hackers Contest Makes a Mess of Internet [Sify] · Hacking Contest Threatens Web Sites [Information Week] · Microsoft Launches Digital ID Software [CNet] submit | more ...

What's New on ACM Queue · A Conversation with Jim Gray · How Much Storage is Enough? · You Don't Know Jack about Disks · Programming Without a Net · Web Services: Promises and Compromises

ATA versus SCSI

ATA and SCSI have much in common and use essentially the same addressing model, but they have subtle protocol differences that reflect the differing applications of the two interfaces. [The most important differences have nothing to do with the interface, but everything to do with the head disk assembly (HDA), which the interface is connecting to the system. A complete discussion of these differences is available in "ATA versus SCSI: More Than an Interface," by Dave Anderson, Erik Riedel, and Jim Dykes, FAST, 2003.]

Both interfaces typically use a 512-byte sector. SCSI supports other sector sizes, including incrementally longer lengths. These are used extensively in disk arrays, where the host may read and write 512-byte sectors, but the array controller reads and writes perhaps 520 or 528 bytes, as shown in Figure 9. (The drives have to be formatted for this longer sector.) The controller will append data information it uses to validate the sector—both its address and contents. This prevents any firmware error or electronics bug in either the drive or array controller from corrupting the data without the array being aware of it. This is a key feature in enterprise subsystems, which feature the highest level of reliability and data integrity.

Figure 9

SCSI interfaces (Fibre Channel, parallel SCSI, and serial attached SCSI) allow multiple hosts to connect to a drive. This creates more complexity for the drive firmware, but enables fault-tolerant configurations—i.e., having no single point of failure. It also entails some special commands. SCSI protocols have, for example, reserve and release commands that let a host take exclusive control of a drive when necessary. These are essential to managing a multi-host storage system. Again, this is functionality that would not be justified in a drive for a low-end array or a personal computer, but more than worth the cost in a system that is keeping a business up and running.

The SCSI interface includes other management features that allow more detailed control over the drive's operations. The respective standards committees, T10 for SCSI [www.t10.org] and T13 for ATA [www.t13.org], are good sources for information.

Comment on this Article in the ACM Queue Forums
Previous Page (5/7) Reliability and Performance	Next Page (7/7) The Most Pressing Issue and what else Might Change