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Home > About SAS Hard Drives
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| SAS Hard Drive Technology versus older drive technology
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What is SAS Technology?
Serial Attached SCSI (SAS) is a faster data transfer technology process developed to move data to and from computer storage devices such as hard drives and tape drives at a much faster rate than prior technology such as the parallel SCSI bus technology that first started being used in the mid 1980s. SAS technology is a point-to-point serial protocol that replaces and uses the standard SCSI command set and is "downwards"-compatible with second generation SATA drives. SATA 3.0 Gbit/s drives may be connected to SAS backplanes, but SAS drives may not be connected to SATA backplanes
SAS (Serial Attached SCSI) compared to original parallel SCSI
SAS supports several thousand devices through the use of expanders while Parallel SCSI is limited to just 8, 16, or 32 devices per single channel.
SAS uses a significantly higher transfer speed (1.5 - 3.0 Gbit/s) than the majority of parallel SCSI standards. The speed is optimized on each connection, allowing a much higher data speed versus the shared parallel SCSI across the entire multidrop bus.
The SAS bus is point-to-point while the original SCSI bus is multidrop. For example: each SAS device is connected by a dedicated link to the initiator,and one expander is used. Even if one initiator is connected to the one target, there is no opportunity for disruption; however, with regular parallel SCSI, even this could cause a disruption in data speed.
SAS doesn't require terminator packs like parallel SCSI does for unused connectors. Note: SAS controllers are required to support SATA devices.
SAS Hard Drives versus SATA Hard Drives
SATA simulates the ATA command set and can only support hard drives and CD/DVD Rom drives. However, SAS can also support numerous other devices such as scanners and printers. However, this advantage can also be a minimal detail, since most of these devices today also have alternative connections such as USB, IEEE 1394 (FireWire), and Ethernet.
However, SAS technology allows multipath I/O to devices while SATA does not. However, the newer SATA II makes use of port multipliers to achieve port expansion. In addition, some manufacturers of port multipliers have implemented multipath I/O leveraging port multiplier hardware.
SATA is considered to be the general-purpose successor to Parallel ATA, while the faster (and more expensive) SAS is marketed for critical server applications.
SAS error recovery and reporting utilize SCSI commands which have more functionality than the ATA SMART commands used by SATA drives.
SAS uses higher signaling voltages, allowing SAS to use cables much longer than SATA's limitation of cable length.
Finally, in general most SAS drives provide Tagged Command Queuing, while most newer SATA drives provide Native Command Queuing, each of which has its pros and cons.
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