ReadyBoost

ReadyBoost (codenamed EMD) is a component of Microsoft Windows, first introduced with Microsoft's Windows Vista in 2006 and bundled with Windows 7 in 2009. It works by using flash memory, a USB flash drive, SD card, CompactFlash, external hard drive or any kind of portable flash mass storage system as a drive for disk cache.

ReadyBoost is also used to facilitate SuperFetch, which allows it to perform analysis of boot-time disk usage patterns and creates a cache which is used in subsequent system boots.

Overview
Using ReadyBoost-capable flash memory (NAND memory devices) for caching allows Windows 7 and 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. Flash devices typically are slower than a hard disk for sequential I/O so, to maximize performance, ReadyBoost includes logic that recognizes large, sequential read requests and has the hard disk service these requests.

When a compatible device is plugged in, the Windows AutoPlay dialog offers an additional option to use the flash drive to speed up the system; an additional "ReadyBoost" tab is added to the drive's properties dialog where the amount of space to be used can be configured. 250 MB to 256 GB of flash memory can be assigned (4 GB in the x86 versions of Vista). ReadyBoost compresses and encrypts, with AES-128, all data that is placed on the flash device; Microsoft has stated that a 2:1 compression ratio is typical, so that a 4 GB cache could contain upwards of 8 GB of data.

According to Jim Allchin, for future releases of Windows, ReadyBoost will be able to use spare RAM on other networked Windows PCs.

For a device to be compatible and useful it must conform to these requirements: Other considerations:
 * The removable media's capacity must be at least 256 MB (250 MB after formatting). Windows Vista x86 & x64 is limited to using 4 GB; this restriction has been removed in Windows 7.
 * Windows 7 allows up to eight devices for a maximum of 256 GB of additional memory.
 * The device must have an access time of 1 ms or less.
 * The device must be capable of 2.5 MB/s read speeds for 4 KB random reads spread uniformly across the entire device, and 1.75 MB/s write speeds for 512 KB random writes spread uniformly across the device.
 * Vista SP1's ReadyBoost supports NTFS, FAT16 and FAT32. Windows 7 also supports the new exFAT file system. Vista SP2 does not support ReadyBoost for exFAT file system. Due to the fact that ReadyBoost cache is stored as a file, one has to format flash drive as NTFS or exFAT to use more than 4 GB of space for caching, because FAT16 and FAT32 impose file size limit of 2 and 4 GB respectively.
 * The initial release of ReadyBoost for Windows Vista supports one device. Windows 7 supports multiple flash drives for ReadyBoost.
 * Because ReadyBoost stores its cache as a file rather than directly using the flash device in a raw manner, that file system must be mounted and assigned a drive letter. Simply mounting as a subfolder of another drive won't suffice, as only the root folder of a drive is suited for ReadyBoost cache — otherwise the “ReadyBoost” tab will not appear in the logical volume properties, nor will any previously created cache file be used.
 * Microsoft recommends the amount of flash memory for ReadyBoost acceleration be one to three times the amount of random access memory (RAM) in your computer. This recommendation should not be confused with the message that is displayed in the “ReadyBoost” tab of drive properties dialog: for example, for a flash drive of 16 GB capacity formatted as FAT32 it will display a message that “Windows recommends reserving 4094 MB for optimal performance” even if RAM size is 10 GB, just because 4094 MB is the maximum file size on a FAT32 volume; after reformatting it as NTFS or exFAT, the message changes to “Windows recommends 15180 MB”.
 * If the system drive is a solid state disk (SSD), ReadyBoost is disabled since it would have little or no effect.

Depending on the brand, wear and tear from read-write cycles, and size of the 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.

Performance
A system with 512 MB of RAM (the bare minimum for Windows Vista) can see significant gains from ReadyBoost. In one test case, ReadyBoost speeds up an operation from 11.7 seconds to 2 seconds (increasing physical memory from 512 MB to 1 GB without ReadyBoost reduced it to 0.8 seconds, though).

The core idea of ReadyBoost is that a flash drive has a much faster seek time (less than 1 ms), allowing it to satisfy requests faster than a hard disk when booting or reading certain system files. It also leverages the inherent advantage of two parallel sources from which to read data. Unfortunately, USB 2.0 flash drives are slower for sequential reads and writes, compared to modern desktop hard drives. Desktop hard drives can sustain anywhere from 2 to 10 times the transfer speed of USB 2.0 flash drives but are equal to or slower than USB 3.0 and Firewire (IEEE 1394) for sequential data. So, all USB 2.0 and newer flash drives hold an advantage in random access times: typically around 1 ms, compared to 12 ms and upwards for desktop hard drives. In addition, USB 3.0 and Firewire may also hold a slight advantage on sequential data.

On laptop computers the performance shifts more in favor of flash memory, laptop memory being priced higher than that for desktop systems, and with many laptops using relatively slow 4200 RPM and 5400 RPM hard drives. Additionally, on a laptop, 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 it is a more power efficient way of increasing equipment productivity.

In versions of Vista prior to SP1, ReadyBoost failed to recognize its cache data upon resume from sleep, and restarted the caching process, making ReadyBoost ineffective on machines undergoing frequent sleep/wake cycles. This problem was fixed in Vista SP1.

Note
Since flash drives wear out after a finite (though very large) number of writes, ReadyBoost could eventually wear out the drive it uses&mdash;though this may take a long time, depending on various factors. According to Microsoft, the drive should be able to operate for at least ten years. However, current day flash drives may be delegated toward ReadyBoost in the future as newer capacities will become standard by dwarfing older flash drives.

As pointed out in Mark Russinovich's Inside the Windows Vista Kernel: Part 2, ReadyBoost caches all writes to the local hard disk: "the Ecache.sys device driver intercepts all reads and writes to local hard disk volumes (C:\, for example), and copies any data being written into the caching file that the service created". Experiments show that ReadyBoost may not cache reads when Superfetch is turned off. Since random read is slow for hard disks, performance boosts can be realized when ReadyBoost has expected data to read from. Thus, with Superfetch turned on, pre-populating data into ReadyBoost cache, the performance boost can be much higher than when Superfetch is turned off.

Microsoft links

 * Windows 7 features: ReadyBoost
 * Windows Vista: Features Explained: Windows ReadyBoost
 * Windows Vista Help Page on ReadyBoost
 * Windows Vista: Features Explained: Performance Features (of which ReadyBoost is one)
 * Channel9 interview with Michael Fortin on ReadyBoost and other performance technologies in Windows Vista
 * Section STORAGE-009 in Windows Logo Program Requirements Suite, Version 3.09

Other links

 * Microsoft's Software Patent on ReadyBoost via google
 * AnandTech: Windows Vista Performance Guide - ReadyBoost, ReadyBoost Performance Analysis, and Hard Drive Performance and ReadyBoost
 * How to use and configure Readyboost
 * In depth Tom's Hardware article with analyses of ReadyBoost and Superfetch
 * How to use ReadyBoost guide with pictures
 * Use any USB drive or memory card with ReadyBoost on Vista
 * ReadyBoost Monitor

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