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Not all SATA is alike

Have you seen how SATA-based disk systems have taken over the computing world by a storm within the past two years? Actually, even less. I think the drivers are well understood. What is less understood, is that not all SATA systems are alike. In this column, I'll outline the minimum level of functionality you should look for even in SATA-based systems.

Let's start with the drivers; these issues are very familiar. Compliance pressures. The need to keep data online, in some cases for years or decades. RTO and RPO pressures. Disaster recovery initiatives. All requiring that data be kept on disk. If you had to do all this using regular SCSI or FC disks, you couldn't justify it economically. Not a chance.

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Enter SATA. All of a sudden you have disk drives that cost one fourth of their SCSI or FC counterparts and they come in large capacities that make them economical for applications mentioned above. They are not as fast and they are not as reliable, but they are just fine for these applications. Things like archiving and backup to disk, in other words, secondary usage of disk. Heck, they are good enough even as primary storage for tier-two or tier-three applications. Whether vendors wanted SATA to play in all these areas or not, you have found a way to deploy them. Everywhere. Sometimes, even in places they do not belong. That is where my concern lies. Without question, SATA is a paradigm-shifting technology. It couldn't have arrived soon enough. But it is not right for everything.

Let's get some facts on the table. SCSI or FC drives today are generally about 150 GB, 15,000 RPM, 3ms seek, 1.5 million hour MTBF at 100% duty cycles. SATA drives today are 400-500 GB, 7500 RPM, 9ms seek and shy of 1 million hours MTBF at low (20%) duty cycles. There are many other differences, but this is what most of you look at when making decisions about SATA's use. I recommend that you to consider at least a few more. See, the problem is that mission-critical applications, whether in mid-size or larger companies, all require high availability, as well. Small companies' management would even argue that the same rules apply to them as well. Vendors will tell you not to worry because SATA systems are RAID protected so a single disk drive failure does not cause access issues. Keep in mind that if you put SATA systems in the wrong environment, you risk dual-drive failures (and therefore, data loss) and you risk frequent application degradation while rebuilds are happening. Dual-parity RAID becomes a must in these environments. All systems in an enterprise need a minimum availability requirement. And, in my opinion, many SATA systems on the market today do not meet these minimum requirements.

Here are additional factors to consider. SATA drives are single ported. In order to meet enterprise-level standards, even for secondary applications, I think they need to be dual ported. Fortunately, companies such as Sierra Logic and SiliconStor have developed an inexpensive adjunct card called an interposer that makes a SATA drive dual ported. When used in conjunction with appropriate FC to SATA routers, or SAS expanders, additional reliability and availability features get added to the point that, other than performance and MTBF differences that are inherent in SATA drives, (and of course, cost) they become equivalent to their FC or SCSI cousins. That makes them a solid player in the enterprise. I believe this type of functionality will become integrated into SATA drives, especially enterprise-level products. But that will take a few years. For now, look for SATA systems that have these implemented as adjunct cards. The additional cost is small and they resolve the single port issue, so they are well worth it.

Note that I did not say that, with this new functionality, SATA becomes equivalent to FC or SCSI (now SAS) drives and therefore you should use them for all applications. Far from it. Even for the appropriate applications, like active archiving, backup and restore, or tier-two applications (as primary storage), SATA systems need to have a minimum level of functionality in the enterprise. So when you look at SATA systems, don't think one system is as good as the next. Demand a minimum level of functionality even in SATA systems. Or pay the price later.

About the author: Arun Taneja is the founder and consulting analyst for the Taneja Group. Taneja writes columns and answers questions about data management and related topics.

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