Articles: Storage
 

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Hard disk drives have become substantially larger and faster recently. The amount of their cache memory is now calculated in tens of megabytes. Their traditional parallel interfaces ATA and SCSI have been ousted by the more perfect and faster serial interfaces SATA and SAS. While SATA was entering the market in a slow and steady way, SAS had a different story. It was kind of a ghost interface once: there were SAS controllers and SAS drives, but they had no popularity. The standard itself was free from any deficiencies. As opposed to the incompatible parallel interfaces (the desktop PATA was mechanically incompatible with the server SCSI), SAS controllers supported SATA drives, and the interface itself was handier due to slim cables you can lay out in your system case easily. SAS also offered increased bandwidth, of course. So, the only problem was the high price of SAS devices. The market of high-performance architectures is conservative (because equipment upgrades and errors are costly) and there was no real need for a really high bandwidth at the time the SAS interface appeared first. Therefore the new interface lacked the attention it deserved.

The situation was changing as controllers were getting cheaper, HDDs were getting faster, and the existing equipment was getting outdated. And at one moment the market reached a critical point after which SAS began a rapid conquest. It turned out that building uniform systems with a common interface was profitable: the same RAID controllers with a SAS interface could be used in top-performance workstations as well as in fault-tolerant storage systems based on SATA drives that offered high capacities at low cost. Well, SAS drives can even reached the latter type of systems now. For example, Seagate has announced a 1-terabyte SAS drive in its ES.2 series. In other words, it has a spindle rotation speed of 7200rpm, which is radically different from a typical SAS device that usually makes 10 or 15 thousand rotations per minute.

But let’s get back to our controllers and tests. The four Western Digital Raptor 2 drives with a spindle rotation speed of 10,000rpm that we used in our tests finally proved to be unable to load modern multi-port controllers seriously. The sequential speeds were too low and the amount of operations per second was not high enough to load the controllers’ processors. So, we decided to upgrade our testbed to keep up with the times. The new testbed has not changed much over the older one, but we now use eight Fujitsu MBA3073RC hard drives with a spindle rotation speed of 15,000rpm, 16MB of cache memory, a capacity of 73.5GB, and a SAS interface. Eight such drives deliver very high performance as you will see shortly and can put a heavy load on the controller.

Now let’s take a look at the first RAID controller we are going to test on the new testbed. It is an 8-port SAS RAID controller from Promise that belongs to the company’s newest product series.

 
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