Windows Administration: Inside the Windows Vista Kernel: Part 2

Windows Administration: Inside the Windows Vista Kernel: Part 2:

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ReadyBoost

The speed of CPUs and memory are fast outpacing that of hard disks, so disks are a common system performance bottleneck. Random disk I/O is especially expensive because disk head seek times are on the order of 10 milliseconds-an eternity for today's 3GHz processors. While RAM is ideal for caching disk data, it is relatively expensive. Flash memory, however, is generally cheaper and can service random reads up to 10 times faster than a typical hard disk. Windows Vista, therefore, includes a feature called ReadyBoost to take advantage of flash memory storage devices by creating an intermediate caching layer on them that logically sits between memory and disks.

ReadyBoost consists of a service implemented in %SystemRoot%\System32\Emdmgmt.dll that runs in a Service Host process, and a volume filter driver, %SystemRoot%\System32\Drivers\Ecache.sys. (Emd is short for External Memory Device, the working name for ReadyBoost during its development.) When you insert a flash device like a USB key into a system, the ReadyBoost service looks at the device to determine its performance characteristics and stores the results of its test in HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\Currentversion\Emdmgmt, seen in Figure 1.

Figure 1 ReadyBoost device test results in the registry (Click the image for a larger view)

If you aren't already using a device for caching, and the new device is between 256MB and 32GB in size, has a transfer rate of 2.5MB/s or higher for random 4KB reads, and has a transfer rate of 1.75MB/s or higher for random 512KB writes, then ReadyBoost will ask if you'd like to dedicate up to 4GB of the storage for disk caching. (Although ReadyBoost can use NTFS, it limits the maximum cache size to 4GB to accommodate FAT32 limitations.) If you agree, then the service creates a caching file named ReadyBoost.sfcache in the root of the device and asks SuperFetch to prepopulate the cache in the background.

After the ReadyBoost service initializes caching, 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. Ecache.sys compresses data and typically achieves a 2:1 compression ratio so a 4GB cache file will usually contain 8GB of data. The driver encrypts each block it writes 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.