Hands on: Boost system performance using Raid

We then connected a third 300GB disk and configured all three as a 900GB Raid 0 array; this delivered 103.4 and 78.2Mbytes/sec for average read and write speeds respectively and a burst rate of 109.8Mbytes/sec. Raid 1 requires an even number of disks so was unavailable. Three disks is the minimum for Raid 5 and by losing a disk’s worth of capacity, resulted in a 600GB volume. This delivered average read and write speeds of 84 and 8.8Mbytes/sec respectively, and a burst rate of 103.3Mbytes/sec.

Raid 5 suffers from poor write performance due to the calculation and distribution of parity bits, hence the 8.8Mbytes/sec speed. But it delivers good read speeds and redundancy with a modest hit on capacity compared to Raid 1.

Next we connected the fourth disk and repeated the tests for all three types of Raid arrays. With all four configured as Raid 0, the array measured the maximum 1.2TB and delivered average read and write speeds of 104 and 72.8Mbytes/sec respectively, with a 106Mbytes/sec burst rate. Reconfiguring the four disks as Raid 1 again meant a 50 per cent loss of capacity, with 600GB available. It delivered 97.7 and 46.1Mbytes/sec in average read and write speeds respectively with a burst rate of 107.7Mbytes/sec.

Finally, the four disks were reconfigured as Raid 5, again with the loss of one disk’s worth of capacity to deliver a 900GB volume. This provided average read and write speeds of 92.1 and 12.1Mbytes/sec respectively, with a burst rate of 102Mbytes/sec.

You pays yer money...
There was little performance benefit with a four-disk Raid 0 array on the Promise adapter and PCI bus, but it did allow a single massive volume. With four disks in Raid 0, you’re multiplying your chance of failure by four, so it’s not recommended. Four disks configured as Raid 1 provided redundancy at the cost of half the capacity and modest performance. A Raid 5 array with four disks provided redun dancy but with a capacity loss of 25 per cent. Performance was slightly better than with three-disk version but write speeds remained slow.

An alternative worth considering on recent Intel chipsets is Matrix storage, which exploits four disks to offer Raid 0 and Raid 1 partitions for virtual memory and critical data respectively.

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