Windows® Internals, Sixth Edition, Part 2

ReadyBoost is implemented with the aid of a driver (%SystemRoot%\System32\Drivers\Rdyboost.sys) that is responsible for writing the cached data to the NVRAM device. When you insert a USB flash disk into a system, ReadyBoost looks at the device to determine its performance characteristics and stores the results of its test in HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Emdmgmt, as shown in Figure 10-51. (Emd is short for External Memory Device, the working name for ReadyBoost during its development.)

Figure 10-51. ReadyBoost device test results in the registry

If the new device is between 256 MB and 32 GB in size, has a transfer rate of 2.5 MB per second or higher for random 4-KB reads, and has a transfer rate of 1.75 MB per second or higher for random 512-KB writes, then ReadyBoost will ask if you’d like to dedicate some of the space for disk caching. If you agree, ReadyBoost creates a file named ReadyBoost.sfcache in the root of the device, which it will use to store cached pages.

After initializing caching, ReadyBoost intercepts all reads and writes to local hard disk volumes (C:\, for example) and copies any data being read or written into the caching file that the service created. There are exceptions such as data that hasn’t been read in a long while, or data that belongs to Volume Snapshot requests. Data stored on the cached drive is compressed and typically achieves a 2:1 compression ratio, so a 4-GB cache file will usually contain 8 GB of data. Each block is encrypted as it is written using Advanced Encryption Standard (AES) encryption with a randomly generated per-boot session key in order to guarantee the privacy of the data in the cache if the device is removed from the system.

When ReadyBoost sees random reads that can be satisfied from the cache, it services them from there, but because hard disks have better sequential read access than flash memory, it lets reads that are part of sequential access patterns go directly to the disk even if the data is in the cache. Likewise, when reading the cache, if large I/Os have to be done, the on-disk cache will be read instead.

One disadvantage of depending on flash media is that the user can remove it at any time, which means the system can never solely store critical data on the media (as we’ve seen, writes always go to the secondary storage first). A related technology, ReadyDrive, covered in the next section, offers additional benefits and solves this problem.

ReadyDrive

ReadyDrive is a Windows feature that takes advantage of hybrid hard disk drives (H-HDDs). An H-HDD is a disk with embedded nonvolatile flash memory (also known as NVRAM). Typical H-HDDs include between 50 MB and 512 MB of cache, but the Windows cache limit is 2 TB.

Under ReadyDrive, the drive’s flash memory does not simply act as an automatic, transparent cache, as does the RAM cache common on most hard drives. Instead, Windows uses ATA-8 commands to define the disk data to be held in the flash memory. For example, Windows will save boot data to the cache when the system shuts down, allowing for faster restarting. It also stores portions of hibernation file data in the cache when the system hibernates so that the subsequent resume is faster. Because the cache is enabled even when the disk is spun down, Windows can use the flash memory as a disk-write cache, which avoids spinning up the disk when the system is running on battery power. Keeping the disk spindle turned off can save much of the power consumed by the disk drive under normal usage.

Another consumer of ReadyDrive is Superfetch, since it offers the same advantages as ReadyBoost with some enhanced functionality, such as not requiring an external flash device and having the ability to work persistently. Because the cache is on the actual physical hard drive (which typically a user cannot remove while the computer is running), the hard drive controller typically doesn’t have to worry about the data disappearing and can avoid making writes to the actual disk, using solely the cache.

Unified Caching

For simplicity, we have described the conceptual functionality of Superfetch, ReadyBoost, and ReadyDrive independently. Their storage allocation and content tracking functions, however, are implemented in unified code in the operating system and are integrated with each other. This unified caching mechanism is often referred to as the Store Manager, although the Store Manager is really only one component.