The Promise FastTrak S150 SX4 supports more advanced RAID configurations with three or four disks drives. Having four drives opens up the possiblity of combining striping and mirroring to achieve both high performance and high reliability. One way is to stripe two pairs of disks in RAID-0, and then mirror the pairs in RAID-1. The other way is to mirror the two pairs of disks in RAID-1, and then stripe the mirrored pairs into RAID-0. The first method is called RAID 0+1 (pronounced raid zero plus one) and the second method is called RAID 1+0 (raid one plus zero) or RAID 10 (raid ten).
Both of these four-disk RAID configurations have performance equal to that of a two-disk RAID-0 array. Both of these configurations have the same capacity, which is twice the capacity of the smallest drive. However, their is a distinct advantage to RAID 1+0 over RAID 0+1 when reliability is in question. While both can survive under single-disk failures and some combinations of two-disk failures, RAID 1+0 can survive more combinations of two-disk failures.
More specifically, a four-disk RAID 0+1 array will survive a two-disk failure only if the two drives that failed are in the same striped pair. A four-disk RAID 1+0 array will survive only if the two failed drives are in two different pairs. The difference can be clearly seen in the failure modes table on page 8. Out of six possible combinations of two-drive failures, RAID 0+1 will fail in four while RAID 1+0 will only fail in two. Therefore, strictly speaking, RAID 1+0 is more reliable than RAID 0+1. This is a good way to remember the difference between the two.
The Promise FastTrak S150 SX4, unlike its primary competitors, supports only RAID 0+1 and not the more robust RAID 1+0. Nevertheless, the product specifications list the product as supporting, "RAID 10 -- Data mirrored then striped across four drives, for double drive failure protection" . Indeed, the product manuals and BIOS make no mention of RAID 1+0. This discrepancy between the marketing and actual product capabilities is confirmed by the reviewer at http://www.ebabble.net/promise-s150-sx4.htm (outdated link) ebabble.net.
Nevertheless, the odds of any two disks failing simultaneously in any combination is quite small. The odds decline further if replacement drives are installed promptly after a failure. The Promise FastTrak S150 SX4 controller comes with software to alert you to problems with the array, and recover from problems when they occur. The software, called Promise Array Management, allows one to configure, manage, and monitor a local or remote array.
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