The physical SAS connector is available in several different variants:įorm factor compatible with SATA: allows for SATA drives to connect to a SAS backplane, which obviates the need to install an additional SATA controller just to attach a DVD-writer, for example. SAS supports point data transfer speeds up to 3 Gbit/s, but is expected to reach 12 Gbit/s by the year 2012. The SAS connector is much smaller than traditional parallel SCSI connectors allowing for the small 2.5-inch (64 mm) drives. Because of its higher signaling voltages, SAS can use cables up to 8 m (25 ft) long, SATA is limited to 1 m (3 ft).One reason for this higher voltage is so SAS may be used in server backplanes. When SAS is mixed with SATA, the SAS drives run at SATA-voltages. SAS error recovery and reporting utilize SCSI commands which have more functionality than the ATA SMART commands used by SATA drives.SATA is marketed as a general-purpose successor to Parallel ATA and is now common in the consumer market, while the more expensive SAS is marketed for critical server applications.Some port multiplier manufacturers have implemented multipath I/O leveraging port multiplier hardware. Per specification, SATA II makes use of port multipliers to achieve port expansion. SAS hardware allows multipath I/O to devices while SATA (prior to SATA II) does not.However, this advantage could also be moot, as most such devices have also found alternative paths via such buses as USB, IEEE 1394 (FireWire), and Ethernet. In theory, SAS also supports numerous other devices including scanners and printers. SATA follows the ATA command set and thus only supports hard drives and CD/DVD drives.Most SAS drives provide Tagged Command Queuing, while most newer SATA drives provide Native Command Queuing, each of which has its pros and cons.SATA devices are uniquely identified by their port number connected to the Host bus adapter while SAS devices are uniquely identified by their World Wide Name (WWN).Both SAS and parallel SCSI use the SCSI command-set.SAS controllers are required by the standard to support SATA devices.The speed is realized on each initiator-target connection, hence higher throughput whereas in parallel SCSI the speed is shared across the entire multidrop bus. SAS supports a higher transfer speed (1.5 OR 3.0 Gbit/s) than most parallel SCSI standards.SAS supports up to 16,384 devices through the use of expanders while Parallel SCSI is limited to 8, 16, or 32 devices (including the SCSI controller) on a single channel.SAS has no termination issues and does not require terminator packs like parallel SCSI.If one initiator is connected to one target, there is no opportunity for contention with parallel SCSI, even this situation could cause contention. Each SAS device is connected by a dedicated link to the initiator, unless an expander is used. The SAS bus is point-to-point while the SCSI bus is multidrop.SAS (Serial Attached SCSI) vs parallel SCSI The SAS protocol is developed and maintained by the T10 technical committee of the International Committee for Information Technology Standards (INCITS) and promoted by the SCSI Trade Association (SCSITA). These drives may be connected to SAS backplanes (controllers), but SAS drives can not be connected to SATA backplanes. The protocol is "downwards"-compatible with second generation SATA drives. This will permit much higher speed data transfers. At present it is slightly slower than the final parallel SCSI implementation, but in 2009 it will double its present speed to 6 Gbit/s. SCSI first appeared in the mid 1980s in corporate data centers. It is a point-to-point serial protocol that replaces the parallel SCSI. Serial Attached SCSI ( SAS) is a technology designed to move data to and from computer storage devices such as hard drives and tape drives. Full-duplex with link aggregation (wide ports at 24 Gbit/s)ġ28 device port expanders (16K + total devices)
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