XIV（4）--Flash caching

1，Flash Cache简介

Flash cache 本身是Facebook的一个开源项目，（准确的说是一个Linux的模块），可以动态加载。Flashcache通过在文件系统（VFS）和设备驱动之间新增了一次缓存层，来实现对热门的缓存。Flashcache是另一种缓存，一般用SSD作为介质的缓存（一般的缓存用的是内存），通过将传统硬盘上的热门数据缓存到SSD上，然后利用SSD优秀的读性能，来加速系统。这个方法较之内存缓存，没有内存快，但是空间可以比内存大很多。如下图：

? 现在很多硬件厂商也会在存储设备中增加这个功能来，例如IBM XIV的Flash cache和 EMC的VFCache

2， XIV Flash Cache

从11.1.0开始，XIV Gen3开始支持可选的Flash caching feature，极大的提高小数据块，随机读的性能，适用于Data patterns一直改变的环境。

XIV flash caching is implemented as an extension of the primary cache layer.?? 每个模块400GB Cache，那一个系统就是6TB(400GB*15)，从11.4开始，每个Module能支持800GB。Flash cache 只用于读操作，当不再需要Cache中的数据时，直接drop掉就行了。

XIV flash caching overview

? Flash caching算法是嵌入在XIV Firmware中的，能自动适应相应的IO类型，对用户透明，不需要管理员手动的做performance turning。 ? ?

XIV系统中有2种类型的Cache：main 和 extended ?
--The main cache handles host write I/Os and then destages them directly to the disk drive. //Main cache主要处理主机写IO然后直接Flush到Disk ? ?
--The extended cache handles the caching of random read miss operations less than 64 KB. //Extended cache主要处理小于64KB的随机读操作 ? ?

Sequential read prefetches (larger than 64 KB) are handled in main dynamic random access memory (DRAM) cache. //大于64KB的顺序读操作是由main cache处理的

3，Flash Cache learning

A flash cache map is built as read misses occur in the DRAM cache. The process,known as flash cache learning

关于XIV中Main cache和Extended Cache（Flash Cache）是如何扮演各自角色的，请看下图：

XIV flash cache learning

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The cache node immediately checks the extended cache for the requested I/O. If the requested I/O exists in the extended flash cache,it is served to the host through the main cache. The I/O operation is now complete and is recorded as a flash cache read hit.

If the operation results in a true read miss (not in the DRAM cache and not in extended flash cache),the request is forwarded in an unmodified state to the disk drive (SAS layer). The I/O is retrieved from the disk drive and served to the host through the main cache. From a host perspective,the I/O operation is now complete and is recorded as a read miss. The related pages are copied into reserved buffers in the main cache.

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Important: Any read larger than 64 KB bypasses the extended flash cache.

当Buffer中的data达到512KB时，会顺序地写到Flash cache中。这种方式延长了Flash cache中的寿命。

Note: XIV在系统重启和Firmware升级中能保存Flash Cache中的数据 ? ?

XIV Storage System software Version 11.2 introduced improved flash caching algorithms,providing a performance boost of up to 4.5 times over systems without flash cache for?random database-type workloads. This boost is accomplished by storing and computing all flash cache-related data integrity checking tasks in DRAM rather than on the flash cache.

4，Approaches for Using SSDs in a Storage System

Approach of the tier. With policies that moved the data … or?As a caching layer. This is the approach of XIV.

5，SSD failure

• No redistribution in case of failure/phase-out

• SSD Failure:

? -Reinitialize the metadata of the SSD (so it is not used)
? -The degraded module continue to server reads from its DRAM cache and large sequential reads from its disks
? -Small read misses are redirected to the secondary

• SSD Phase-out and not failed

- Its data is invalidated (on writes)
=> if phased-in not all data is lost

During a rebuild,following a SAS disk failure in a module,the data on SSD in that module is

not invalidated. Rather,it is gradually updated to contain the new data blocks (the same way

the DRAM does).

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后记：关于EMC VFCache和IBM Flash Cache，感兴趣的可以看看比特网的下图对比。

参考文章：http://storage.chinabyte.com/223/12261223.shtml