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.

PC World testing

Back in May, PC World looked at flash drive speed boosting.

The premise was: Although writing data to and reading it from a flash drive is, in most cases, slower than writing and reading to a hard drive, if the data is scattered randomly in small chunks, then flash drives are faster. Vista's ReadyBoost is supposed to use that one speed advantage to create a faster, flash-drive-based cache of one of Windows' major bottlenecks - the swap file on your hard drive that most Windows operations use. So ReadyBoost should theoretically speed up certain frequently performed Windows tasks such as loading programs.

The testing found that none of the devices significantly improved Vista PC performance using ReadyBoost, which is pretty much confirmed by the latest results. It suggested installing more RAM would be a better way of making a Vista PC go faster.

Since these tests nothing much has changed. Flash memory appears not to be an accelerative panacea for slowed-down Vista PCs. Hybrid drives with 250MB or thereabouts of flash cache can help save battery life on Vista and XP notebooks but don't appear yet to increase the speed of PCs running either O/S.

More DRAM could be the answer if the Vista PC has less than 4GB or if it just does a great deal of swapping; add DRAM to help remedy that; ditto for XP up to the 4GB addressing limit of that 32-bit O/S.

XP flash booster

If flash cache speed boosting does work it would be great to have it available for Windows XP instead of using it to overcome users' reluctance to upgrade to Windows Vista. You can try boosting Windows XP PC speed yourself with flash memory by joining a beta test programme.

Software supplier Moscow Development Office (MDO) is beta testing the use of externally-connected flash drives as caches for PC hard drive I/O. Hard drives can be used too but that seems an odd thing to do; use a hard drive as a cache to speed up another ... hard drive.

The company states that 'eBoostr allows you to use an additional drive (flash memory or hard disk) as another layer of performance-boosting cache for your Windows XP ... With the newly developed eBoostr, the booting of your OS and applications start-up get much faster thanks to the smart cashing mechanism. More applications are able to run without accessing the slow hard drive.'

With eBoostr, all you need to boost your computer performance according to MDO is: plug in a flash drive; choose it as a device to speed up your computer; and set the amount of this flash drive's memory space to be used.

MOD says up to four such externally-connected devices can be used as a cache.

The product shows the best results for frequently used applications and data, which becomes a great feature for people who are using office programs, graphics applications or developer tools. That's true in general for any effective cache technology; it's frequently-accessed data that stays in the cache.

If it works then it should be attractive to laptop owners as laptop upgrade is usually more complicated and laptop hard drives are by definition slower than those of desktops. Apply to join the beta programme here.