Why is HDD speed related to ReadyBoost?

This may help to explain Ready Boost & how it is used. This was copied from the Windows Help files.

Using memory in your storage device to speed up your computer

ReadyBoost can speed up your computer by using storage space on most USB flash drives and flash memory cards. When you plug a ReadyBoost-compatible storage device into your computer, the AutoPlay dialog box offers you the option to speed up your computer using ReadyBoost. If you select this option, you can choose how much memory on the device to use for this purpose.

When you set up a device to work with ReadyBoost, Windows shows you how much space it recommends you allow it to use for optimal performance. For ReadyBoost to effectively speed up your computer, the flash drive or memory card should have at least 1 gigabyte (GB) of available space. If your device doesn't have enough available space for ReadyBoost, you'll see a message telling you to free some space on the device if you want to use it to speed up your system.

You can enable or disable ReadyBoost for a specific flash drive or other removable storage device. For more information, see Turn ReadyBoost on or off for a storage device.

Picture of the ReadyBoost tabThe ReadyBoost tab lets you decide how much storage space on a removable device to use for boosting your system speed.

Show contentHide content What to look for in a flash memory device
Here are some tips on what to look for when selecting a USB flash drive or flash memory card to use with ReadyBoost:

The minimum amount of available space recommended for ReadyBoost to effectively speed up your computer is 1 GB.

For best results, use a flash drive or flash memory card with available space of at least double the amount of memory (RAM) in your computer, and preferably four times as much memory. For example, if your computer has 1 GB of RAM and you plug in a 4 GB USB flash drive, set aside at least 2 GB on the flash drive to get the best performance gain from ReadyBoost, and preferably the entire 4 GB. How much memory you need depends on how you use your computer. Keeping a lot of programs open at once uses more memory.

Give ReadyBoost 2 GB to 4 GB of space for best results on most computers. You can reserve more than 4 GB of space for ReadyBoost on most flash drives and flash memory cards. (Storage devices formatted with the older FAT32 file system can't store more than 4 GB.) You can use a maximum of 32 GB of available space on any single removable storage device with ReadyBoost and up to 256 GB total per computer (by inserting up to eight USB flash drives or flash memory cards into the same computer).

To work with ReadyBoost, a USB flash drive must support USB 2.0 or higher. Your computer must have at least one free USB 2.0 port where you can plug in the flash drive. ReadyBoost works best if you plug the flash drive into a USB port directly on the computer, rather than into an external USB hub shared with other USB devices.

If you want to be sure a USB flash drive works with ReadyBoost, look for a note from the manufacturer that the flash drive is "Enhanced for ReadyBoost." Not all manufacturers list this on their packaging. If there is no mention of ReadyBoost compatibility, the flash drive still might work with ReadyBoost.

There are many different kinds of flash memory cards, such as CompactFlash and Secure Digital (SD) memory cards. Most memory cards work with ReadyBoost. Some SD memory cards don't work well with ReadyBoost due to issues with the SD card interface. ReadyBoost will display a warning message if you attempt to use one of these cards.

Notes
If your computer has a hard disk that uses solid-state drive (SSD) technology, you might not see an option to speed up your computer with ReadyBoost when you plug in a USB flash drive or flash memory card. You may instead receive the message, "Readyboost is not enabled on this computer because the system disk is fast enough that ReadyBoost is unlikely to provide any additional benefit." This is because some SSD drives are so fast they're unlikely to benefit from ReadyBoost.

In some situations, you might not be able to use all of the memory on your device to speed up your computer. For example, some flash memory devices contain both slow and fast flash memory, but ReadyBoost can only use fast flash memory to speed up your computer.

Lots of pointless answers that don't address the question

x BlueRobot said:

ReadyBoost is used to improve caching speeds, it is a false popular belief that it provides more RAM or acts as RAM. It primarily uses data in the SuperFetch cache at first, however, later on once all the apportiate ReadyBoost services have been loaded, the Ecache.sys reads all disk I/O to the connected hard-drives, and then begins writing any data from these disk I/Os into a cache file stored on the USB flash drive.

This nails it on the head however. But what exactly is the "cache" in this case? Is it the stuff that gets loaded into SuperFetch? And why does SuperFetch cache to the HDD instead of RAM?

More questions than answers...
I've never bothered with Ready Boost and don't really understand how it could give you much benefit. If you look at the R/W performance of the run of the mill USB flash drive vs a 7200rpm HDD the HDD wins hands down. Most USB flash drives are relatively slow. Run something like CrystalDiskMark on both.

Conventional drives do indeed have a faster transfer rate than a flash drive. The faster transfer rate will be a benefit when doing large sequential reads. But in most real world situations that just doesn't happen very often in a modern OS. Typical usage patterns are small random reads from multiple files. In that case it is seek time that is more important than transfer rate. The primary performance characteristics are usually seek time, rotation delay, transfer rate. In that order. Typically a hard drive will spend more time seeking data than reading it, and by a considerable margin.

Flash drives have slower transfer rate but much better seek times. ReadyBoost takes advantage of this. When doing small random reads ReadyBoost will read the data from the flash drive if it is present there. This takes advantage of the faster seek time of the flash drive and the slower transfer rate doesn't matter much. But when doing large sequential reads the file system will go directly to the hard drive and bypass the flash drive.

The NT platform has long had an efficient system cache for the hard drive. The problem is that the efficiency of the system varies with the amount of RAM available for the purpose. On systems with limited RAM there just isn't enough for a decent size cache.

ReadyBoost was introduced with Vista that had a minimum RAM specification of 512 MB for the home basic edition. Some time ago I read of ReadyBoost testing on such a system. ReadyBoost significantly improved performance, but upping the RAM to 1 GB helped more. Under favorable conditions ReadyBoost could be of some benefit with as much as 2GB RAM.

ReadyBoost was retained in Windows 7 and Windows 8 for situations where it would be a benefit. But on modern systems with 4 GB RAM and more and with fast conventional or SSD drives it benefits nothing.

Ok I'm going to mark this solved then since perhaps only Microsoft truly knows what the hell is going on inside this thing

As far as I can work out HDD speed is in no way related to Readyboost. In other words - using Readyboost doesn't speed up read/ write access. It just allows some frequently used programs to load faster.

On my machine I see no need to use either readyboost or superfetch. I prefer to allocate a chunk of RAM to a persitent RAMdisk. It's configured so that USER temp and WINDOWS temp files are written onto it. This directly relates to HDD speed.

Pagefile is swapped onto a non-windows partition and set to a fixed size of "system recommended plus 200Mb" - the page file on the windows partition is fixed at 16MB only. No need for superfetch.

Screenshot shows Windows partition (C: ) vs RAMdisk (V: ) - an insane speed boost.

ReadyBoost has everything to do with Disk speed. ReadyBoost is essentially a disk cache. A basic concept of caching is that the media used as the cache, a flash drive, must be faster than the media being cached, the hard drive. RAM is also being used as a disk cache but on low memory systems there just isn't enough. A flash drive has slower transfer rate but seek time is much better. The slower the hard disk the more effective ReadyBoost will be.

The benchmarks shown do indeed favor the RAMDisk by a wide margin. But the numbers have little relevance to real world performance. All modern operating systems has a sophisticated system cache that provide most of the benefits of a RAMDisk with fewer of the problems. If files are frequently accessed then they will be in the cache and the speed of the disk is of no consequence except for the initial read. If files are not frequently why would you want then in the RAMDisk? RAMDisks tend to be a poor use of memory. The OS will usually make better use of it.

A pagefile on a non-system partition will impair performance unless it is also on a separate physical drive.