ReadyBoost is a component of Microsoft Windows that was introduced with Windows Vista in 2006 and is also included with Windows 7.
It is included in Windows Vista Editions Home Basic, Home Premium, Business and Ultimate.

Adding to RAM (system memory) is usually the best way to improve your PC’s performance, since more memory means more applications are ready to run without accessing the hard drive. However, upgrading memory can at times be difficult and costly, and some PCs have limited memory expansion capabilities, making it impossible to add RAM.

Windows Vista introduced Windows ReadyBoost, a new concept in adding more memory to a system. It works by using non-volatile flash memory, USB (universal serial bus) 2.0 drive, SD card, CompactFlash or any kind of portable flash data storage device as a drive for disk cache, thus improving the performance without having to add additional memory “under the hood.”

The USB Flash Drive serves as an additional memory cache, memory that the computer can access far more quickly than it can access data on the hard drive. Windows ReadyBoost relies on the intelligent memory management of Windows SuperFetch and can significantly improve system responsiveness.

Windows ReadyBoost is easy to use. When a USB Flash Drive or SD (secure digital) memory card or other removable memory device is first inserted into a port, Windows Vista checks if its performance is fast enough to work with Windows ReadyBoost. Then you are asked if you want to use this portable memory device to speed up your system’s performance. You can allocate only a part of your USB Flash Drive’s memory to speed up performance and use the remainder to store files.
Windows ReadyBoost is also used to facilitate SuperFetch, an updated version of Windows XP’s prefetcher which performs analysis of boot-time disk usage patterns and creates a cache which is used in subsequent system boots.

Using ReadyBoost-capable flash memory drive for caching allows Windows Vista to service random disk reads with performance that is typically 80-100 times faster than random reads from traditional hard drives. This caching applies to all disk content, not just the page file or system DLLs.  USB Flash memory devices are typically slower than a hard disk for sequential I/O so, to maximize performance, Windows ReadyBoost includes logic that recognizes large, sequential read requests and has the hard disk service these requests.

When a compatible removable memory device such as USB Flash Drive is plugged in, the Windows AutoPlay dialog offers an additional option to use the Flash Drive to speed up the systems performance; an additional “ReadyBoost” tab is added to the drive’s properties dialog where the amount of space to be used can be configured. Flash memory drive ranging in capacity from 250 MB to 4 GB to can be assigned. ReadyBoost encrypts, with AES-128, and compresses all data that is placed on the flash memory device. According to Microsoft a 2:1 compression ratio is typical, so that a 4 GB cache could contain upwards of 8 GB of data.
For a USB Flash Drive device to be compatible with Windows ReadyBoost these are the requirements:

• USB Flash Drive minimum capacity after formatting must be 256 MB—250 MB. Windows Vista is limited to using 4GB; this restriction has been removed in Windows 7.
• USB Flash Drive must have an access time of 1 ms or less.
• USB Flash Drive read speeds must be 2.5 MB/s for 4 KB random reads spread uniformly across the entire flash memory device.
• USB Flash Drive write speeds must be 1.75 MB/s for 512 KB random writes spread uniformly across the device.

Other considerations and requirements:

Vista SP1’s ReadyBoost supports NTFS, FAT16 and FAT32.
Windows 7 and Vista SP2 also support the new exFAT file system.
The initial release of ReadyBoost for Windows Vista supports one device. Windows 7 supports multiple flash drives for ReadyBoost.
Microsoft recommends the amount of flash memory for ReadyBoost acceleration to be one to three times the amount of random access memory (RAM) in your computer.
Depending on the brand, wear and tear from read-write cycles, and size of the USB flash memory, the ability to format as NTFS may not be available. Enabling write caching on the flash drive by selecting Optimize for performance in Device Manager allows formatting as NTFS.
ReadyBoost is not available on Windows Server 2008.
System Performance with Windows ReadyBoost
A system with 512 MB of RAM (the bare minimum for Windows Vista) can see significant gains from ReadyBoost technology. In a test case, ReadyBoost improved the operation speed from 11.7 seconds to 2 seconds; increasing physical memory from 512 MB to 1 GB reduced it to 0.8 seconds.

With Windows ReadyBoost a USB Flash Drive has a much faster seek time (less than 1 millisecond), allowing it to satisfy requests faster than a hard disk when booting or reading certain system files. It also leverages the advantage of two parallel sources from which to read data. Compared to modern desktop hard drives, USB Flash Drive devices are relatively slow for sequential reads and writes. Desktop hard drives can sustain anywhere from 2 to 10 times the transfer speed of USB Flash Drives. However in random access times, USB Flash Drives hold an advantage, typically around 1ms, compared to 12ms and upwards for desktop hard drives.

On laptop computers the performance shifts more in the favour of removable USB Flash Memory Drive, with laptop PC memory being priced higher than that for desktop PC systems, and with many laptops using relatively slow 4200 RPM and 5400 RPM hard drives. Additionally, on a laptop PC, ReadyBoost caching can reduce hard drive access, allowing the hard drive to spin down for increased battery life. Also, because of the nature of the power management typically enabled during mobile use of a laptop PC it is a more power efficient way of increasing hardware productivity.

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