It's actually a bit more complicated than SATA and SAS replacing SCSI. Traditional parallel SCSI is a tried-and-true disk interface that's been around for decades. SCSI currently offers very fast burst data transfers of 320 megabytes per second (MBps) using today's 16-bit Ultra320 SCSI interface.
SCSI also offers features such as Tagged Command Queueing (TCQ), to improve I/O performance. SCSI hard drives are noted for their reliability, and it's possible to daisy-chain up to 15 devices per SCSI adapter channel over short distances. These features have made SCSI a good choice for performance-oriented desktops and workstations, all the way up to enterprise-class servers -- even to this day.
SAS drives follow the SCSI command set and carry many of the same characteristics of reliability and performance found in SCSI drives, but they employ a 300 MBps serial version of the SCSI interface. Although this is a bit slower than SCSI at 320 MBps, a SAS interface can support up to 128 devices over longer distances than Ultra320 and can be expanded to 16,000 devices on a channel. SAS adoption still has a long way to go in the enterprise. SAS drives offer the same reliability and the same 10,000-15,000 rpm rotational speeds that SCSI drives do.
SATA drives forego some of that performance and reliability of SCSI and SAS drives in favor of sheer storage capacity and lower cost. For example, SATA drives have now reached 1 TB. SATA has been embraced where maximum storage capacity is needed, such as disk backups and archiving. SATA currently offers point-to-point connections up to 300 MBps, which easily exceeds the traditional 150 MBps parallel ATA interface.
So while SCSI works fine, traditional SCSI is reaching the end of its practical service life. A 320 MBps parallel SCSI interface won't go much faster at the distances of today's SCSI cables. By comparison, SATA drives should reach 600 MBps in the near future, and SAS drives have a roadmap out to 1200 MBps. SATA drives can also run on the SAS interface, so these drives can be mixed in the same storage system. The potential for expandability and data transfer performance is just overwhelming SCSI.
But SCSI isn't going away any time soon. You'll see SCSI linger on small to midsized servers for a few years. As the hardware is updated, SCSI will be systematically replaced by SAS/SATA disk arrays for faster speed and superior connectivity.
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