SATA vs. SCSI vs. SAS
Compare The Major Hard Drive Technologies
Serial ATA has been common technology in desktops and workstations for quite some time now. Market share in server environments has been growing steadily over the last couple of years. The interest for Serial ATA is certainly there. The server market is very conservative by nature, however. Many businesses still prefer SCSI for mission critical appliations. Reliability and availability are top priority. Serial ATA still has a long road ahead in winning the trust of corporate users. There are many true and false stories circling around about the performance of SCSI systems in server workloads. On this page, we will make a direct comparison between SCSI and SATA RAID.
There are many differences in the interface technology and in the commands executed. SCSI has a robust tagged command queuing implementation to allow multiple commands to be outstanding which provides significant performance gains for drives or controllers by being able to order the commands in an optimal execution manner. ATA only allows a bus release that is mistakenly called command queuing. The implementation on the class of disk drives that support ATA is different as well.
ATA drives are cheaper than SCSI or Fibre Channel drives and there's a reason. SCSI and FC drives use a processor for executing the commands and handling the interface and a separate processor controlling the head positioning through servos. ATA drives use a single processor for both which means that if the rotational positioning requires more adjustments due to factors such as rotational vibration or wear, more processor time will be dedicated for that which can affect the performance of the drive. Because of this, you will usually see a lower RPM for the ATA disk drive and also a shorter warranty period. But it is cheaper and can be a very good, economical solution for many environments and applications.
We have no intention to pass sound judgements about the reliability of the Serial ATA solutions reviewed in this article. However, based on the features of the adapters in this test we can say that SATA RAID need not be inferior compared to alternatives using a SCSI interface. The high-end SATA RAID adapters are supporting all the important availability features found on SCSI RAID-adapters. The SATA RAID-adapters made by Areca and LSI Logic have been directly ported from architectures which have been used in SCSI RAID systems for many years. So with respect to features, Serial ATA is certainly up to the job of handling enterprise storage.
That leaves the question if Serial ATA can offer acceptable performance in workstation and server environments. Thanks to the large number of RAID adapters which have been tested by Tweakers.net in the previous months, we can certainly say something meaningful about the performance of SATA and SCSI RAID systems. Besides performance, price and rack density are important selling points that need to be taken into consideration. Does one give priority to the highest performance or the lowest price/capacity, or is performance and capacity per height unit important? These different priorities leave us with several possible interpretations of the results you find on this page. For example, a SATA RAID setup using six 10.000rpm hard drives can be cheaper and better performing than a SCSI RAID setup, whereas SCSI setups with 15K hard disks will realize a higher performance per height unit. No discussion is needed about the answer to the question which configuration provides the lowest price per gigabyte. Serial ATA will always have the advantage in this area.
SATA configurations perform admirably well in file server and streaming media scenarios, while SCSI configurations (dependent on the specific type of adapter and harddisks) are king of the hill in database applications. With the arrival of Serial ATA II all shortcomings of the ATA-interface concerning performance and reliability have been solved. A new generation of hard disks, supporting native command queuing, will be able to offer even better performance than the Raptor WD740GD? in this comparison. At the same time, the arrival of Serial Attached SCSI and future SAS-harddisks with spindle speeds of 22,000rpm ensure that the SCSI interface will consolidate its unique featureset and its performance advantage in certain workloads. SCSI, thanks to its higher level market position, will always be the technology to profit first from technological advancements. These technological advantage will not only affect harddisk performance, but will also have its effect on RAID adapter performance. The higher priced SAS RAID solutions will be equipped with the latest generation I/O processors and larger amounts of cache memory, for instance. In the low-end server market - single and dual processors sytems using direct attached storage - Serial ATA will become a mature and powerful alternative to SCSI.