ATA Standards
| Today, the ATA interface is controlled by an independent group of representatives from major PC, drive, and component manufacturers. This group, called Technical Committee T13 (www.t13.org), is responsible for all standards related to the PATA and SATA storage interfaces. Technical Committee T13 is a part of the InterNational Committee on Information Technology Standards (INCITS; www.incits.org), which operates under rules approved by the American National Standards Institute (ANSI; www.ansi.org), a governing body that sets rules that control nonproprietary standards in the computer industry as well as many other industries. A second group, called the Serial ATA Working Group (now the Serial ATA International Organization [SATA-IO; www.serialata.org]) was formed to initially create the SATA standards, which are then passed on to Technical Committee T13 for refinement and official publication under ANSI. The current ATA-7 standard incorporates both PATA and SATA standards and represents the end of the road for PATA because ATA-8 and beyond will contain only SATA standards. The rules under which these committees operate are designed to ensure that voluntary industry standards are developed through the consensus of people and organizations in the affected industries. INCITS specifically develops information processing system standards; ANSI approves the process under which they are developed and then publishes them. Because Technical Committee T13 is essentially a public organization, all the working drafts, discussions, and meetings of Technical Committee T13 are open for all to see. Copies of any of the published standards can be purchased from ANSI or Global Engineering Documents (see the vendor list in Appendix C, "Vendor List"). Draft versions of the standards can be downloaded from Technical Committee T13 and SATA-IO websites. The ATA interface has evolved into several successive standard versions:
Since ATA-1, newer versions of the ATA interface and complementary BIOS have supported larger and faster drives, as well as different types of devices other than hard disks. ATA-2 and later have improved the original ATA interface in five main areas:
Each newer version of ATA is backward compatible with the previous versions. In other words, older ATA-1 or ATA-2 devices work fine on ATA-6 and ATA-7 interfaces. ATA-7 includes both PATA and SATA, but SATA-8 is serial only. Newer versions of ATA are normally built on older versions and with few exceptions can be thought of as extensions of the previous versions. This means that ATA-7, for example, is generally considered equal to ATA-6, with the addition of some features. Table 6.1 breaks down the various ATA standards. The following sections describe all the ATA versions in more detail.
Virtually all recent servers support ATA-6 or newer ATA standards, so the following sections discuss these standards in greater detail. ATA-6/ATAPIThe ATA-6 standard includes Ultra ATA/100 (also called UDMA/100), which increases the Ultra ATA burst transfer rate by reducing setup times and increasing the clock rate. As with ATA-5, the faster modes require the improved 80-conductor cable (refer to Figure 6.6, later in this chapter, for a comparison of the original 40-conductor and current 80-conductor PATA cables). Using the ATA/100 mode requires both a drive and a motherboard interface that supports that mode. Work on ATA-6 began in 2000, and the standard was finished and officially published in 2002, as ANSI NCITS 361-2002, "AT Attachment - 6 with Packet Interface." The major changes or additions in the standard include the following:
Besides adding the 100MBps UDMA Mode 5 transfer rate, ATA-6 also extended drive capacity greatlyand just in time. ATA-5 and earlier standards supported drives of up to only 137GB in capacity, which became a limitation because larger drives were becoming available. Commercially available 3.5-inch drives exceeding 137GB were introduced during 2001, but they were originally available only in SCSI versions because SCSI doesn't share the same limitations as ATA. With ATA-6, the sector addressing limit was extended from 228 sectors to 248 sectors. This means that LBA addressing previously could use only 28-bit numbers, but with ATA-6, LBA addressing can use larger 48-bit numbers, if necessary. With 512 bytes per sector, this raises the maximum supported drive capacity to 144.12PB, which is equal to more than 144.12 quadrillion bytes. Note that the 48-bit addressing is optional and necessary only for drives larger than 137GB. Drives 137GB or smaller can use either 28-bit or 48-bit addressing. ATA-7/ATAPIThe primary addition in ATA-7 is another transfer mode for PATA, called UDMA Mode 6, that allows for data transfers up to 133MBps. As with UDMA Mode 5 (100MBps) and UDMA Mode 4 (66MBps), the use of an 80-conductor cable is required. Slower speeds don't require the 80-conductor cable, although they do work with it and an 80-conductor cable is always preferred over the 40-conductor type. Another major change in the specification is the inclusion of the SATA 1.0 specification into ATA-7. This makes SATA an official part of the ATA standard. Work on ATA-7 began in 2001, and the standard was finished and officially published in 2004. Note that although the throughput has been increased from the drive controller (on the drive) to the motherboard via the UDMA modes, most ATA driveseven those capable of UDMA Mode 6 (133MBps) from the drive to the motherboardstill have an average maximum sustained transfer rate while reading data of under 60MBps. This means that although newer ATA drives can transfer at speeds up to 133MBps from the circuit board on the drive to the motherboard, data from the drive media (platters) through the heads to the circuit board on the drive moves at less than half that rate. For that reason, running a drive capable of UDMA Mode 6 (133MBps) on a motherboard capable of only UDMA Mode 5 (100MBps) really doesn't slow things down much, if at all. Likewise, upgrading your ATA host adapter from one that does 100MBps to one that can do 133MBps doesn't help much if your drive reads data off the disk platters at only half that speed. When selecting a drive, remember that the media transfer rate is far more important than the interface transfer rate because the media transfer rate is the limiting factor. ATA-8In 2004, work began on SATA-8, which is a new ATA standard based on ATA-7. The following are the main features of SATA-8:
The most dramatic change is the development of separate PATA and SATA standards. ATA8 for PATA drives is known as ATA8-APT (ATA/ATAPI Parallel Transport), while ATA8 for SATA drives is known as ATA8-AST (ATA/ATAPI Serial Transport). The main benefit of this change is to allow new features and functions to be made available in ATA8-SPTcompliant SATA drives while providing a PATA-specific standard that can be used as a reference. By creating separate standards for PATA and SATA, the result is a clearer and more concise description of the SATA standard. ATA8-APT is virtually identical to the PATA-specific content of ATA-7. Basically, any PATA drive compliant with ATA-7 is also compliant with ATA8-APT. It is expected that both versions of the ATA8 standard will be finalized and officially published in 2006. |
EAN: 2147483647
Pages: 240