Quiet lately

The reason I haven't been posting much is that I haven't been up to much here at work. Most of my projects are waiting on other people to get done before I start. I found a really nifty tool that I want to try out a few times before I proclaim it to the heavens, so that's waiting on the right error condition before I do so.

In other news, there was a Slashdot article yesterday along the lines of, "What RAID, JBOD, or Whatnot should I use for my home storage center?"

There are two questions that drive my answer to this overall question:

1) Is the capacity you are shooting for larger than a single drive?
2) How important is write speed?

If you're looking at 1TB of space, you can do that several ways:

• Buy a 1TB drive
• Buy 2 500GB drives and use RAID0 to span them
• Buy 3 500GB drives and use RAID5 to span them
• Buy 2 1TB drives and RAID1 them
• Buy 4 320GB drives and use RAID5 to span them
• Buy 4 500GB drives and use RAID0+1 to span them

How important write-speed is to you will determine whether or not RAID5 is a good fit for you. As I've shown in previous benchmarks, even on enterprise SAN hardware the parity calculation limits how fast data can be committed to disk. I don't have benchmarks that compare, say Linux LUM RAID5 versus a hardware RAID5 controller to see which writes faster with a fast CPU so that will depend. In general, it is better to parallelize that where possible, which means a separate controller that performs the parity computation independent of the main CPU. For a media-center PC this means you can use your whole CPU to transcode that incoming raw video stream into MPEG4, without having to worry about CPU contention for writing to disk.

That said, the PCI-Express SATA RAID controllers that I can find on NewEgg all use software Raid5 through the storage driver when they support it. If you go PCI-X, that changes and you have several options in the $200-$400 range that will do true hardware RAID5.

Newegg puts a Seagate Barracuda 7200.10 500GB drive at $124, the 320GB of the same product line at $90, and the 1TB Hitachi Deskstar drive at $400.

• ($400) Buy a 1TB drive
• ($250) Buy 2 500GB drives and use RAID0 to span them
• ($375+$250 = $625) Buy 3 500GB drives and use RAID5 to span them
• ($800) Buy 2 1TB drives and RAID1 them
• ($360+$250 = $610) Buy 4 320GB drives and use RAID5 to span them
• ($500) Buy 4 500GB drives and use RAID0+1 to span them

In the above case, it is actually cheaper to get 4 500GB drives and use software RAID0+1 than it is to purchase an additional RAID controller with RAID5 offload, at least for the 1TB size. The down side is you're installing 4 drives in your case and that may push the limit to what you can fit. This is probably why the hardware RAID controllers don't really exist in the PCI-Express market, which is predominantly a desktop bus not a server bus.

Whether or not write performance is a big issue for you will tell you whether or not spending $375 for a software RAID5 makes sense over spending $250 for a non-redundant RAID0. How disastrous a hard-drive crash will be will tell you whether or not to spend the extra $250 for a redundant RAID0+1 setup.