Slowed-down PCs, bottlenecked by tardy disk I/O, are supposed to be getting a speed booster, using hybrid disks and Windows Vista. The latter's ReadyBoost feature would cache frequently-accessed data in a flash memory cache inside a hard drive enclosure, thus getting data into RAM faster.

Intel also promoted the concept of having a flash cache on a PC's motherboard, the Robson technology, to achieve the same end.

That was the theory. However, recent results suggest that the flash cache boost is illusory.

Flash and hard drive I/O characteristics

The background here is that flash memory I/O isn't simply faster than disk in general. Flash is generally faster than disk for small and random read I/O operations but slower than disk for small write I/O operations. Generally every write has to be preceded by san erase cycle. Also different flash vendors use different write algorithms and designs and have significantly varying write speeds: one flash size does not fit all here.

Flash is not faster than disk for mid-range and large I/Os, both writes and reads.

This means that flash cacheing has to have good algorithms for what to load into cache and also that the pattern of I/O has to have a large proportion of small and random read I/Os for the flash cache to significantly boost PC operating speed. In the real world that may well not be the case.

There are quite significant differences in operations between external ReadyBoost USB flash, Robson motherboard flash, and hybrid hard drives with their internal flash. Robson and ReadyBoost use the same thing: external flash to the hard drive, to cache temporary read and write I/Os and so avoid writing them to disk. When Vista wants data it looks in both the Robson/ReadyBoost cache and on disk and gets the data from the first to respond.

Hybrid drive flash caches everything passing to/from that hard drive. It is also both more expensive and faster than the kind of flash seen in USB thumb drives. It's purpose is twofold: to deliver data faster; and to enable the hard drive to spin down and save power and thus lengthen battery life.

On desktop PCs battery life isn't an issue.

The amount and type of cache in a hybrid drive can make a telling difference. Japanese supplier M-Cell has added a 1GB DRAM cache to a 2.5-inch hard drive. This makes a 5400rpm 2.5-disk perform faster than a non-cache 7200rpm 3.5-inch hard drive.

DRAM is much faster than NAND or OR flash and also more expensive.