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SubjectAnnouncement: STEC EnhanceIO SSD caching software for Linux kernel
Greetings,

STEC is happy to announce hosting of our EnhanceIO SSD caching software on github.
We would like to invite kernel hackers to try it. We'll appreciate your valuable feedback to help us improve it to the standards of Linux kernel source code. We hope to eventually submit it for a possible inclusion in Linux kernel.

Repository location - https://github.com/stec-inc/EnhanceIO
License - GPL
Source - Derived from the source base of EnhanceIO product Current state - Alpha.
Ongoing work - Code cleanup, testing, more documentation.

Do try it. If you face problems, file bugs at github or write to me.

First section of the README.txt file in this repository introduces EnhanceIO and is as follows

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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.
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Look forward to hearing from you.
Thanks.
--
Amit Kale

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