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Adaptive Algorithms

For random accesses, a new adaptive algorithm uses the history of previous requests to assign read operations in a way

that minimizes the movement of the disk arm. These optimizations have shown superlinear performance gains on random

read operations. Superlinear means that performance gains are better than linear, with two drives giving greater than two

times the performance of one drive. In the results shown in Figure 2, the performance gain is about 2.3 times that of a single

drive. The reason for this outstanding result is that there are twice as many actuators and each travels less distance than the

average distance when only one drive is used.

In most RAID 1 configurations, each I/O is directed to one of the disks and there is no performance improvement if small

fixed-length stripes are read alternately from the two drives. For instance, if disk 0 reads the even 32K stripes and disk 1

reads the odd 32K stripes, both disks transfer half the time and spend the other half of the time waiting for the head to pass

over data being read by the other drive. This phenomenon is shown on the left side of Figure 2 with small stripe sizes. As the

stripe size is increased, it eventually passes the point where the amount of data being skipped is equal to one track.

At this point the data rate increases sharply because there is almost no time wasted for the head to pass over data being

transferred by the other drive. At the first peak, the data rate is not quite equal to reading the drive sequentially because one

extra disk skew is required when skipping the track read by the other drive. Later peaks have higher bandwidth because the

extra skew is spread across more tracks of transferred data. The position of the peaks and the performance at each peak

vary depending on the bit density, RPM of the drive and the particular zone being measured.

Figure 2. Performance Sensitivity to Stripe Size

The storage switch takes advantage of this phenomenon by setting a stripe size at one of these peaks and simultaneously

accessing alternating stripes from the two drives. In this way, long sequential reads run at nearly twice the rate of a single

drive. The peaks shift to the left at each zone crossing when moving from the outer diameter of the disk toward the inner

tracks. For optimal performance, the zone table is consulted at each zone crossing in order to set the stripe size to the

optimal value for that zone. The combination of the sequential and random access methods gives improved performance

over a wide range of applications