Storage Interfaces SATA, SCSI, SAS, FC - part2

ATA (Advanced Technology Attachment):
ATA interface introduced in the mid 1980’s, is the primary internal storage interface for the PC, connecting the host system to peripherals such as hard drives, optical drives, CD-ROMs.
ATA is also called as PATA because it is Parallel Interface.
The latest version of the PATA is Ultra ATA & supports up to maxium of 100Mbytes/sec data transfer rates using double edge clocking.
ATA supports upto 2 interface connections, that means only two devices can be connected.
ATA is being replaced by SerialATA.

SATA (Serial ATA):
The SATA interface is an evolution of the ATA interface from parallel bus to serial bus architecture. The serial bus architecture overcomes the difficult electrical constraints hindering continued speed enhancement of the parallel ATA bus.
The first generation SATA-I technology is designed to be a direct serial transport mechanism for ATA protocol data at 150Mbyte/sec that is fully compliant with the ATA protocol at the software level.
SATA II is Structured and prominent protocol enhancements to further speed increases to 300 Mbyte/sec (3Gbit/s) and to improve the protocol efficiency in a multitasking environment.
With these enhancements, the SATA would approach the performance of SCSI/FC-based disk drives.

SCSI (Small Computer System Interface):
SCSI is also Parallel interface technology and SCSI has gone through a long evolution (from 1980s), with SCSI-I, SCSI-2, and SCSI-3 over different types of cabling, and including Wide SCSI and Narrow SCSI.
The primary benefits of SCSI include: cross platform interoperability; support for a large number of devices; easy expandability; long cabling distances for external device connectivity;
very good support for multitasking disk accesses that allow for interleaving of multiple concurrent transfers; tag queue and out-of-order data delivery.
SCSI interface supports up to seven devices with Narrow SCSI, and 15 devices with Wide SCSI.
SCSI products tend to cost quite a bit more than the ATA/SATA disk drives for the same disk capacity.
SCSI evolution (SCSI1/FastSCSI/WideSCSI/UltraSCSI) has also improved its data transfer speed from 5MB/s to 320 MB/s.

SAS (Serial Attached SCSI):
the Serial Attached SCSI (SAS) is being defined to replace the physical layer of SCSI i.e parallel to serial bus/interface technology.
The goal is to achieve higher data transfer rates and yet maintain the protocol compatibility of SCSI at the command set level.
Currently SAS supports upto 300 MB/s and work is in-progress for development of 600MB/s & higher speed.
SAS uses a serial, point-to-point topology and hence can connect 2 devices in direct connection and also using SAS Expander(hub) can connect upto 128 devices.
SAS & SATA interfaces looks very similar and with Expanders, SAS & SATA devices can be connected to similar ports using a device called 'Interposer card'. thus making huge advantage of different interfaces co-exist in same Storage box (devices like SAS Enclosure).

Note: SAS interface is primarily Direct Drive interface like SCSI & SATA but using SAS expanders(hub) & SAS Switches can be Networked as 'SAS Domain' and hence can also used as Interconnection topology.

Firbe Channel:
FC is High speed Serial Interface, prominent interface since its existence from 1990s.
Like SCSI, FC is also has well defined Protocol structure and promising capabilities compared to other interfaces.
Like S.A.S, FC interface has Point-point topology, Loop topology(FC-AL), Fabric-Switched topology. P-p supports 2 devices, Loop supports 126 devices & Switched fabric supports upto 16 million devices.
FC started with data transfer speed of 100MB/s (1Gbit/s), 300MB/s,400MB/s & plans for 600MB/s to 1000MB/s and hence most of the Networked Storage environment (SAN) uses FC technology.

Note:
* Interfaces like ATA/SATA/SCSI/SAS are majorly used as drive-side interfaces in DAS & SAN environments.
* FC is not only used as Drive-side interface in DAS but also, pre-dominantly used Drive-side & Host-side interface in SAN Environment.
* For more information you can access www.SNIA.org, www.t10.org (for SCSI interface) & www.t11.org (for FC interface).
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