Articles: Storage

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The corporate sector, which is the main consumer of such serious products as RAID controllers, is highly conservative. For a business, any failure or downtime, however small they may be, can provoke a financial loss. Most consumers in this market follow the principle of making haste slowly. It is easy to come up with an example: while desktop HDDs quickly transitioned from the parallel to serial interface (from PATA to SATA), the similar transition from SCSI to SAS has taken a long while. Disk racks with SCSI 320 interface are not any kind of museum rarity even today. Anyway, it is time for yet another change. This time around, the SAS interface has improved its bandwidth from 3Gbps to 6Gbps. The new SAS 2.0 standard has brought about some other innovations including reduced EMI, an increased maximum length of cables (from 6 to 10 meters) with reduced crosstalk, and a revised topology of complex configurations. It is the double bandwidth that is the most prominent and called-for innovation, though. Of course, the new standard is fully compatible with the old one. The transition would be much more difficult otherwise. Generally speaking, SAS has a stable niche among today’s disk interfaces. It is “shorter” than the competing Fibre Channel and iSCSI but much cheaper than the former and has lower latencies than the latter. Now at 6Gbps, SAS looks like a very nice way of connecting disks located in several adjacent racks.

As a matter of fact, the bandwidth of 3Gbps (or, theoretically, 300GBps) is already quite enough if each disk is connected to a dedicated port of the RAID controller. Even the best of today’s SAS drives, the models with a spindle rotation speed of 15,000rpm and a huge recording density, are only approaching a read speed of 200MBps. So why is the higher bandwidth so important? Because it can be used not only for connecting individual disks but whole racks of disks! If one controller’s channel is shared by multiple drives, the bandwidth of 3Gbps is not enough as it can only satisfy two modern HDDs. We shouldn’t also forget about solid state drives that easily deliver speeds of 250-270MBps and are already limited by the interface.

It is also easy to tell what users need this increased bandwidth. It is not necessary for those who want as many operations per second from their disk subsystem as possible, but it is vitally important for users who need to quickly read and write large amounts of data simultaneously. Various file storage systems or video-on-demand servers are examples of that.

Now, let’s have a look at the first SAS RAID controller we’ve got that supports SAS 6Gbps. It is the MegaRAID SAS 9260-8i model from a new controller series introduced by the renowned LSI.

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