Serial Attached Small Computer System Interface (SAS) is the next generation enterprise-class serial storage technology. It replaces parallel SCSI, which reached the end of its technology roadmap with Ultra 320 after serving many low- and mid-range RAID applications.
SCSI suited users who did not need or could not afford the extreme performance and reliability of Fibre Channel.
SAS, like SATA, uses a serial interconnect. The SAS interface is an evolutionary superset of the SATA standard but you cannot simply say it is better than the alternative.
The two serial technologies have different advantages and selecting the best disk interface, serial infrastructure and RAID controller interface for your particular application is key.
SAS is designed to serve datacentre and server environments for expanded connectivity, data availability and the highest levels of random performance.
It can also compete with costly Fibre Channel in high-end environments.
In most storage applications where aging parallel SCSI infrastructures are to be displaced, choose the respective benefits of SAS and SATA.
SATA offers high capacity, low cost-per-GB and sustained data rates.
SAS has greater expansion capability and higher random access performance for database applications.
The compatibility between SAS and SATA changes the way integrators can think about systems. Systems don’t need to be purely SAS or SATA: in some applications integrators will choose to mix SATA drives in a SAS infrastructure.
This creates a system that benefits from the expandability of SAS and the low cost-per-GB and the high capacity potential of SATA. This ability to choose SAS, SATA, or a hybrid approach offers greater system design flexibility.
SATA will remain the cost and performance-optimised interface for most cheaper storage appliances, particularly if they have a fixed number of drives to serve vertical applications, such as video-on-demand systems, disk-to-disk backup systems and video surveillance recording.
These applications are characterised by sequential disk transactions, matching the strengths of SATA drives.
SAS expandability is not often needed, particularly those targeted for vertical markets where users may have limited knowledge of storage technologies, as they are preconfigured to meet the predetermined demands of particular applications.
SAS will emerge to capture the direct-attach server space, SCSI’s former mainstay. Mission-critical applications that require higher levels of availability, native expansion and higher random access performance will increasingly rely on SAS.
SAS also offers the ability to easily expand capacity with the use of standard SAS expanders.
The main trade-off that system integrators need to make is between cost and performance.
In performance-driven applications, SAS offers higher spin speeds and much higher random access performance.
Whether to use direct-connect or a SAS backplane with expanders is determined by the number of drives required, with expanders providing a far more effective solution than using multiple RAID controller cards.
When integrators need to achieve a balance between cost and performance – for example in a SAN device or a storage appliance – SATA’s cost-per-Gigabyte will make it the best choice for the application.
Smaller storage appliances with 24 or fewer drives are better served by a lower-cost SATA controller and a direct connect infrastructure. In larger appliances, or SAN solutions that need expansion capabilities, a SAS controller in a SAS infrastructure provides the expandability and flexibility required.
Much of the storage market is about cheap solutions with no requirement for system expansion: NAS systems and many vertical markets such as video on demand, CCTV and disk-to-disk backup, for example.
These applications are often characterised by sequential data access. A SATA infrastructure, with SATA controllers and drives, provide the most cost effective and sometimes the highest performance solution.
Choosing the right disk technology, infrastructure and RAID controller is crucial if integrators are to develop the best solution to meet a customer’s specific requirements. SAS is not a ‘one size fits all’ proposition.
Ron MacLeod is sales director for Europe at AMCC/3Ware
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