Server Tech

A note on Server Storage – SAS (Serial Attached SCSI), SAS Controllers & SAS Drives

Serial Attached SCSI (SAS) has become the favorite interface for enterprise server storage (hard drives/ SSD’s). In this article, we will explore the advantages of SAS to understand the reason behind their popularity. We will also shortly explore about SAS controllers and SAS drives.

What is Serial Attached SCSI (SAS)?

Serial Attached SCSI (SAS) is a storage interconnect fabric that connects the enterprise hard drives/ SSD’s with the server motherboard. You might have heard of SATA Hard Disks (Parallel ATA earlier) in your Laptop/ Computer. Similarly, parallel SCSI which was earlier popular with enterprise servers are being replaced by serial technology called SAS. There are a number of advantages of using serial technology instead of parallel technology for SCSI (as given below) but the main reason for the shift is to accommodate higher performances.

Advantages of Serial Attached SCSI (SAS):

  • Cables, connectors, HBA, backplane, RAID controllers, etc are similar for both SAS and SATA. So, its possible to plug in SATA based drives along with SAS drives in to the same SAS backplane. This enables the usage of the low cost/ high volume SATA drives along with SAS drives for a few applications. But some vendors do not support certain features like mixing SAS and SATA drives in the same RAID array.
  • SAS supports interconnect speeds of 3 Gb/s & 6 Gb/s today with a road map to support 12 Gb/s. All of them are backward compatible.
  • SAS has a higher reliability and performance than SATA. Some SAS Controllers support disk speeds up to 15,000 RPM in addition to 10,000 & 7200 RPM, for better performance.
  • Since SAS uses serial point to point technology, each port gets a dedicated route/ bandwidth and the cables are thinner than parallel SCSI and SATA.
  • SAS is a Direct Attached Storage interface and can expand to accommodate external JBOD enclosures, as well.
  • SAS supports Dual Port technology which provides two separate data paths per drive. This eliminates single point of failure allowing for fail over in the other path with full bandwidth. This enhances their reliability.
  • SAS utilizes the small 7-pin connector and the cable supports distances of up to 8-10 meters. The earlier parallel interfaces usually don’t support more than 1 meter.
  • SAS uses the same SCSI command set and hence applications written for parallel SCSI would work with SAS as well.
  • Each SAS port requires lower power than the corresponding power requirement for parallel SCSI port.
  • SAS is built for 24×7 duty cycle, and hence is suitable for enterprise server storage requirements.
  • SAS has provisions for data integrity checks with IOEDC.

Serial Attached SCSI (SAS) Controllers:

A SAS Controller (Also known as RAID Controller) is required for managing multiple SAS/ SATA drives in the Server/ external JBOD’s. It is also required for implementing RAID redundancy option for disk drives. The common RAID configurations supported are RAID 0, RAID 1, RAID 10, RAID 5, RAID 6 but not everything is supported by every vendor and every model. SAS Controllers may have integrated RAID controller chip module.

A SAS Controller has a certain fixed number of internal ports – Commonly 8 Ports/ 16 Ports, which can connect to as many SAS/SATA drives. It also has certain external ports through which many more disk drives can be supported through external JBOD’s. An eight port SAS Controller for example, can have a maximum of eight disk drives directly attached to it. But through the external ports and expander units, the number of SAS disks attached to it can go as high as 16,000.

The SAS Controller is attached directly to the server motherboard by some vendors (or) is attached in the PCI Express expansion slots in the server motherboard by others. Some SAS controllers even support hot swappable disks.

Serial Attached SCSI (SAS) Drives/ Disks:

SAS Disks/ Drives are available in two main form factors – 3.5″ and 2.5″ SFF (Small Form Factor). Generally, the 3.5″ SAS drives are available in various sizes from 80 GB to 2 TB while the 2.5″ drives are available from 50 GB to 1 TB (The capacities mentioned here are indicative and depends on the manufacturer/ model). Generally, the 7200 RPM rated drives support the highest storage capacities and the 15,000 RPM rated drives support the highest performance.

SAS technology supports variable sector sizes. Some SAS drive vendors also support encryption technology to safeguard the data stored in them.

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