RAID/HBA Controllers

SAS RAID Controller, is it right for your application?


SAS RAID controller is on the top of Anova Microsystems’ recommendation list for high-end server and workstation, but why? The answer is definitely not trivial in that we cannot merely say “because it is better". In order to comprehend the subject a little better one must trace and see the progression of the concept of a RAID and implantation and to how it relates now to storage and data management redundancy in workstation, database, and enterprise computing.

The term RAID (in reference to SAS RAID controller) was presented by David Patterson, Garth Gibson and Randy Kats in 1988 in a conference article “A Case for Redundant Arrays of Inexpensive Disks (RAID).” In more recent times, the acronym RAID may also be referred to as a Redundant Array of Independent Disks as to reveal that the cost of individual hard disk drives is in itself a low-cost item. In the article by Patterson, Gibson, and Kats, they outlined various levels in a numerical format. This nomenclature for RAID still holds today with the most popular and recognized RAID levels, 0 (no redundancy, and sometimes considered a stripe), 1 (mirror) 5, (stripe with parity, and 6 (stripe with double parity). Without confusing and or otherwise getting too technical, each RAID level has its merit and advantages, however with the case of RAID level 0, if a stripped drive fails, all data on the volume is lost. RAID level 1 will have a mirror but a performance hit is taken when writing to two simultaneous drives. Most common for Enterprise Storage application using a SAS RAID controller would utilize hardware RAID 5, or RAID 6. RAID level 5 consumes at least two disk drives where data and a parity block are written over all drives. In the case of a hard disk failure, the RAID 5 volume may still be operational until a new replaced drive is inserted and the data volume is allowed to rebuild. One downside of using a SAS RAID controller in RAID 5 mode is if there are two concurrent drive failures, the entire RAID volume is lost. In RAID level 6, double data parity is written over several drives so it can withstand two concurrent drive failures within a given volume. The minimum number of hard disk drives used in a SAS RAID controller and for that fact a SATA RAID controller is four drives.

The term SAS, is referred to in a SAS RAID controller, has been used without much justification and explanation. SAS is an acronym for Serial Attached SCSI. The term SCSI is another acronym for Small Computer Systems Interface and uses legacy parallel data bus transfer. That is, data in and out to various drives on the SCSI chain runs in parallel where each drive is identified by the SCSI ID number. Due to the increasing need of larger drives and the need for more drives in a system, there was a practical limit of about 15 devices on a SCSI data chain to which even lead to signal degradation, SCSI termination restrictions, as well as overall throughput performance.
Due to the abovementioned problems with traditional SCSI, industry leaders like Adaptec, LSI, among others, looked to transition parallel data transfer to a serial type interface. Serial transfer technology transmits data in a stream of data like a water pipe in which it is not bound by clock limitations allowing for greater data streams and thus allowing for better reliability. SAS (Serial Attached SCSI) was developed to address the need for high-performance and reliable data transfer. According to an Adaptec whitepaper, “SAS is a high-performance solution that leverages proven SCSI functionality, and builds on the enterprise expertise of multiple chip, board, drive, subsystems, and server manufacturers throughout the industry.” Key features when using a SAS RAID controller include:
  • The ability to use both SAS and SATA drives within one controller. This gives customers a choice to which type of drives one wishes to implement connected to their SAS RAID controller.

  • A reliable point-to-point connection with speeds now reaching 6Gb/s and up to 16,256 addressable devices within a SINGLE SAS port

  • Enterprise-class features including NCQ (native command queuing) and staggered spin up

  • Thinner and more manageable cabling within a subsystem to allow better airflow and maintenance.

These major features coupled with name-brand recognition from Adaptec, LSI, and Areca, leverage this exciting technology within SAS RAID controllers to power storage systems and workstations for years to come.