Back-end switching won’t have any effect on rebuilt times as you will still be reconstructing to a single drive. That means that reconstruction can not be faster that the write throughput of a single disk.

Our HDS team *casually* mentioned that 750GB ATA drive support for the USP-V will be announced Monday October 10th and will be available on November 14th. 1TB ATA soon to follow. I almost fell out of my chair!

With this in mind, how would you qualify your above statement that “Intermixing, slower, less reliable SATA or FATA disks in tier 1 storage systems will impact that system’s performance and availability with more frequent drive failures and longer rebuild times which consumes internal bandwidth, cache, and controller cycles. For these reason’s we do not offer FATA disks in our USP/NSC, preferring to attach them through external disk systems.”

The use of semiconductors for storage would result in reduction of power and heat.

So it seems the ideal would be to create a true back-end fabric (as opposed to loop) architecture so there is no performance or availability reason to copyback. Then just specify a number or percentage of disks in the spare pool. If a disk fails, replace that physical disk. Anything else is needless complexity which should have been automated out of storage arrays a decade ago.

Next, regarding the capacity vs. I/O cost tradeoff, if capacity is cheap, but I/Os are expensive, a return to RAID 1 (or 1+0) semantics may be in order. At least narrower stripes may be in order, especially for SATA. Some customers are doubling their stripe width with RAID6 compared to RAID5, so as to keep their capacity costs constant.

Maybe it is time to consider a 4-disk wide RAID 6 stripe: The same capacity as mirroring, but you still have a RAID 5 level of protection with a single disk failure. However, many fewer I/Os are required to rebuild a disk. Imagine a system which immediately block copied the entire affected LUN to a new LUN, rather than a spare-in of disk. This would allow full-stripe writes to the new volume instead of Read Modify Writes to the spare. Spare volumes rather than spare disks are certainly possible if capacity is cheap.

Or maybe the answer is in RAID 1 with an automated ability to access a DR or HSM copy if a second drive fails. The performance of a long-wave accessed copy, or a SATA copy after a FC drive failure, may be similar to accessing a degraded RAID5 or RAID6 volume. This would quite literally be “thinking outside of the box”, with respect to the storage array.