Oracle pushes low-cost Apple disks

Oracle is collaborating with the new Apple Xserv RAID devices, a low-cost data storage array with hardware RAID. This disk array features “hot-swappable” disk “modules, and the device can hold up to 7 terabytes. Life after Coffee’s Jon Emmons notes that the cost is $12k, but that is only $1.86 per gig. The Apple pitch notes:

“the gigabyte-per-dollar ratio of Xserve RAID is the best in the world for Fibre Channel storage and trumps most SCSI storage solutions as well. Xserve RAID offers up to 7TB of high-performance storage at under $2 per gigabyte — a fraction of the cost of storage from Dell, HP, Sun or IBM.”

However, it’s the speed of the disk that counts, not the cost, and here is the spec sheet for the Xserv disks. The first scary part is that each spindle has an 8 meg cache, a component that suggests highly variable access speed.

It turns-out that the disk spindles are 250 gig or 500 gig behemoths, and there are serious speed issues associated with super-large disks in Oracle databases. If your whole database fits on a single spindle, the device can shake like an out-of-balance washing machine when the read-write heads get moving, especially if you have popular data files on the outer cylinders:

I always get suspicious when I don’t see average speed numbers (instead they use throughput numbers), so we have to wonder “what is the average access latency” of these spindles?

How fast is the Apple XServ RAID?

Oracle DBA's are interested in average access speed, not throughput numbers, and we have to do some math to guesstimate the average latency of the Apple XServ RAID disks.

Ironically, I could only find the average seek delay on an eBay ad: “Average seek time - 8.5 ms”. We can compute the average rotational delay based on the spin speed (7,200 RPM), assuming that. on average, we have to wait for "half a revolution". Hence, the average rotational latency is as follows:

7,200 RPM = 120 revolutions per second

Hence, we have one revolution every 1/120th of a second = .00833 = 8.33 milliseconds per revolution.

Hence, the average rotational delay for this device (assuming that must wait for half a revolution) is 4.16 ms.

OK, now that we know the average seek and rotational latency, we can finally get the average latency, excluding data transmission time:

Average seek delay =             8.50 ms
Average Rotational delay =   4.16 ms
                                              12.66 ms

Of course, the total average speed will be reduced by the on-board 8 meg data caches, but we still have the problem of minimizing the seek delay. If we place frequently-accessed data files on the outer cylinders, the average seek delay will be far longer:

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