Articles: Storage

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What do people do when they run short of the performance, capacity or reliability of even the best hard disk drive? They turn to RAID technology. People who need increased reliability go for RAID1. Users who have lower speed and reliability requirements but demand high storage capacity and low cost of storage prefer RAID5 or RAID6 (the latter is a variant of RAID5 but with two checksums per each stripe). RAID0 is for speed. Both speed and reliability can be ensured by using RAID10 or, less frequently, RAID50 and RAID60, which are two-layered combinations of the basic arrays. It is all quite simple and you can easily select a suitable RAID type for any application, from workstations and entry-level servers to high-performance servers with lots of HDDs inside.

One more thing to keep in mind is that the cost of building a RAID is the cost of the HDDs plus the cost of the RAID controller. If you make a complex array out of a lot of disks, you need an appropriate, i.e. expensive, controller. The price factor is not so important if you are building a data storage system on the enterprise level, but it is a serious factor for those who are going to use their RAID in a workstation or in a simple server at a small firm.

There are three ways to lower the cost of the array:

  • Use fewer disks in it
  • Take cheaper disks (e.g. cheaper SATA instead of SAS disks)
  • Use an inexpensive RAID-controller

It is all clear about item one: there is a minimum number of disks you need to build each type of RAID. You are not absolutely free to choose the amount of disks, though. After all, this determines such parameters as the capacity and speed of the array (the speed parameter changes especially greatly in the case of RAID0).

There is not much freedom about item two of the list: the quality of HDDs affects the speed and capacity of the array, too.

So, we’ve got to item three. Why can one controller be cheaper than another? It may have a slower processor, a smaller amount of cache memory (or no such memory at all whereas top-performance controllers come with as much as 512 megabytes of it), and fewer HDD connectors (this simplifies the PCB wiring and lowers the requirements to the peak bandwidth of the controller’s internals).

If you’ve got no special requirements about your RAID and you don’t expect the disk load of your server to increase greatly in near future, you can reduce the cost of the array significantly by preferring a simpler RAID controller.

You may be wondering why I am telling you such simple truths. Well, it’s just I am about to offer you a comparative review of a few different RAID controllers from Adaptec. Let’s find out what applications each of them suits best.

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