EnhanceIO driver is based on EnhanceIO SSD caching software product
 developed by STEC Inc. EnhanceIO was derived from Facebook's open source
 Flashcache project. EnhanceIO uses SSDs as cache devices for
 traditional rotating hard disk drives (referred to as source volumes
 throughout this document).

 EnhanceIO can work with any block device, be it an entire physical
 disk, an individual disk partition, a RAIDed DAS device, a SAN volume,
 a device mapper volume or a software RAID (md) device.

 The source volume to SSD mapping is a set-associative mapping based on
 the source volume sector number with a default set size
 (aka associativity) of 512 blocks and a default block size of 4 KB.
 Partial cache blocks are not used. The default value of 4 KB is chosen
 because it is the common I/O block size of most storage systems. With
 these default values, each cache set is 2 MB (512 * 4 KB). Therefore,
 a 400 GB SSD will have a little less than 200,000 cache sets because a
 little space is used for storing the meta data on the SSD.

 EnhanceIO supports three caching modes: read-only, write-through, and
 write-back and three cache replacement policies: random, FIFO, and LRU.

 Read-only caching mode causes EnhanceIO to direct write IO requests only
 to HDD. Read IO requests are issued to HDD and the data read from HDD is
 stored on SSD. Subsequent Read requests for the same blocks are carried
 out from SSD, thus reducing their latency by a substantial amount.

 In Write-through mode - reads are handled similar to Read-only mode.
 Write-through mode causes EnhanceIO to write application data to both
 HDD and SSD. Subsequent reads of the same data benefit because they can
 be served from SSD.

 Write-back improves write latency by writing application requested data
 only to SSD. This data, referred to as dirty data, is copied later to
 HDD asynchronously. Reads are handled similar to Read-only and
 Write-through modes.