FAST Cache for 统一存储 VNX

1. FAST Cache简介1.1 固态硬盘(SSD)新玩法关于什么是固态硬盘，参考《维基百科》与《百度百科》。

2008年，首个尝试将固态硬盘带入企业级存储系统的EMC，受到了业界极大的质疑，普遍的观点，甚至包括许多专业人士和存储媒体都认为，固态硬盘加入企业级存储系统在当时是个“天方夜谭”：固态硬盘的寿命太短了，写入速度的提高也不如读取速度提高一样显著，最关键的问题是，固态硬盘的价格太贵了，贵到连许多财大气粗的企业都难以承受。

EMC并不否认人们的普遍看法，但却仍然坚持自己对于固态硬盘的“偏执”。

随后的两年，在固态硬盘的销售上EMC尝尽甜头，而许多供应商也相继在存储系统上提供了固态硬盘。

随着存储分层技术和Cache技术的发展，如今，EMC让固态硬盘成为了更具实用性的产品。

虽然实现方式与此前已经上市的基于固态硬盘技术的PCIe存储系统加速卡略有不同，但固态硬盘成为存储系统“标配”的时代现在看来已经来临。

固态硬盘的“新玩法”就是EMC的FAST VP和FAST Cache技术。

其实CLARiiON 的FLARE 30就已经发布FAST VP与FAST Cache，关于FAST VP请参考《FAST VP for 统一存储 VNX 》，本文将探讨FAST Cache技术。

1.2 什么是FAST Cache在《FAST VP for 统一存储 VNX 》讲到，实现 AST 有两种不同方式：迁移与缓存。

基于缓存的 AST 使用广为人知的缓存方式将热数据“提升”到高性能的介质中。

由于 HDD 上仍保留有数据的副本，因此当数据变“冷”时，只需将其从缓存中释放即可，而不需要额外的 HDD I/O。

对应于VNX的FAST Cache。

简单说来，FAST Cache就是把系统内空闲的SSD（EMC称之为EFD），用来替代CLARiiON系统上的内存，当做一种数据高峰时在高速的系统内存底下一层的缓存来使用，而作为内存的补充，由于SSD也都是基于FLASH技术实现的，因此，速度也确实能够允许其在内存和磁盘间作为缓存来用。

2. FAST Cache与FAST VPFAST Cache与FAST VP最大的区别是其重新构建一个能满足高性能要求的Flash drive存储Cache区,重点关注性能，而FAST VP只是通过Pool来实现sub-LUN的分层，从而实现数据根据性能要求分布到存储的不同层，关注总使用成本(TCO)。

EMC建议整合FAST、FAST Cache和块级压缩技术，提升中端系统的性能：其区别如下：1.FAST VP最小单位是1 GB，而FAST Cache是64KB。

2.当数据被频繁访问时，将其从硬盘拷贝到Flash drives中，而FAST VP只是在不同层间移动.3. FAST Cache可以与FAST VP结合起来一起使用,比如，当数据已经正在移向或已经在Flash drive时就不必将其移动到FAST Cache，即使该数据块满足要提升到FAST Cache条件，从而避免浪费系统资源。

4.一般推荐是在存储中的Flash drive首先作为FAST Cache,但在某些情况下，如当IO块小于128KB时，因其读64KB需要多个IO次数来完成其提升时，建议用FAST VP方式5. 当FAST Cache处于degrade状态时，FAST Cache会变成只读。

性能层会通过快速提升64KB大小的chunk到FAST Cache,从而减少数据到FAST Cache的时间。

总结:从数据的角度来看，一般情况下，整个存储中只有一小部分空间提供了绝大多数的IO.数据分析表明一个卷的85%的IO量是由其15%的容量完成的。

这个15%的容量称贷为”工作区”,FAST VP和FAST Cache就是将这些”工作区”尽可能地保存在高性能的磁盘中。

OLTP环境下，”工作区”一般只占20%或更小的容量。

而对于DSS来说”工作区”会占大得多的容量。

这时一般就推荐用FAST VP而不是FAST Cache.3. FAST Cache 常见限制与要求∙It can be used on all CLARiiON CX4 storage systems running FLARE release 30or later. If you have a Celerra unified storage system, Data Movers need to be running Celerra version 6.0 or later (along with a back-end CX4 running FLARE 30) to see FAST Cache benefits.All models in the VNX series support FAST Cache.∙It leverages the strong history of Flash drives and uses existing Flash drives, Disk Array Enclosures, and back-end buses. Forexample, the Flash drives do not need to be inserted in a particular drive slot or on dedicated back-end buses.∙ A single FAST Cache instance per storage system serves as a system-wide resource. You can create and destroy the FAST Cache while the storage system and applications are online.∙When you create a FAST Cache, the Flash drives are configured as RAID 1 mirrors (redundant).∙Once a Flash drive has been used to configure FAST Cache, the entire Flash drive is dedicated to FAST Cache.∙FAST Cache can be applied to any LUN in a storage system. If a LUN (or component LUNs of a metaLUN) is created in a RAID group, FAST Cache is enabled or disabled at the individual LUN (or component LUN) level. If a LUN is created in a storage pool, FAST Cache must be configured at thepool level.4. FAST Cache 剖析4.1 FAST Cache 组件4.2 FAST Cache 工作原理FAST Cache不中：If FAST Cache is installed in the storage system and enabled on the LUN and/or pool, incoming I/O from the host application is checked against the FAST Cache memory map to determine if the I/O is for a data block that has already been promoted into FAST Cache or not. If the chunk is not in FAST Cache, the I/O request follows the same path it would follow if the storage system did not have FAST Cache.FAST Cache 命中：However, if the data chunk is in FAST Cache, the policy engine redirects the I/O request to the FAST Cache. If the host I/O request is for a read operation, and the target data is in the DRAM cache, the data is read from the DRAM cache. If the data is not in DRAM cache, the data is read from the FAST Cache Flash drives; it is also put in the DRAM cache (as it would be with reads from HDDs).If the host I/O request is a write operation for a data chunk that is in FAST Cache, the DRAM cache is updated with the new “write,” and an acknowledgement is sent back to the host. If DRAM write cache is not disabled for the HDD LUN, write operations usually go through the DRAM cache; the host data is not written directly to the FAST Cache Flash drives. When data needs to be moved out of DRAM cache, it is written to FAST Cache Flash drives. Since the data is written to Flash drives instead of HDDs, this operation is very fast and may help limit the number of dirty pages in the DRAM write cache. It should be noted that even when FAST Cache is installed in the storage system, the I/O operations are serviced directly from DRAM cache whenever possible.Data that is in FAST Cache and is used less frequently is copied back to the HDD from FAST Cache. This chunk of data must be re-promoted into FAST Cache to be serviced by the FAST Cache. Data on HDD that becomes busy is promoted to FAST Cache, which is an asynchronous process (does not block host I/O). Data promotion into FAST Cache depends on the number of accesses (read and/or write) within a 64 KB chunk of storage, and is not dependent on whether the data already exists in the DRAM cache or not.For example, assume a storage system receives an I/O request from an application right after FAST Cache is created on the storage system. In this scenario, the FAST Cache memory is empty since nothing has been copied into the FAST Cache.∙When the first I/O is sent by the application, the FAST Cache policy engine looks for an entry in the FAST Cache memory map for the I/O’s data chunk . Since the memory map is empty at this stage, the data is accessed from the HDD LUN. This is called a FAST Cache Miss. (We have found that the performance overhead of checking the memory map for every access to a FAST Cache enabled LUN is very minimal.) ∙If the application frequently accesses data in a 64 KB chunk of storage, the policy engine copies that chunk from the hard-disk LUN to FAST Cache. The memory map is updated to indicate that the data chunk is now resident in FAST Cache. This operation is calledPromotion and this period is called thewarm-up period for FAST Cache. The storage system controls this activity to make sure that the overhead for promotion does notexceed a specified percentage of the storage system capability.When the application accesses this data again, the policy engine sees that it is in the FAST Cache. This is called the FAST Cache Hit. Since the data is accessed directly from the Flash drives, the application gets very low response times and high IOPS. If a substantial part of the working set gets promoted to FAST Cache overa period of time, applications can see higher average performanceeven with lower performing HDDs in the back end.In certain situations, data is copied from FAST Cache back to the back-end HDDs. This is called a Write Back operation. The rate at which data blocks are written back to the HDD LUNs is controlled so that the process does not significantly impact storage system performance. Write Back operations happen in the event that a FAST Cache promotion is scheduled but there are no free or clean pages available in the FAST Cache. A “dirty” page will then be copied from the FAST Cache and written to the HDD LUN to make room for the new data. The LRU algorithm determines which data block is least likely to be accessed again and this block is processed to make room for the new promote.5. FAST Cache 的使用5.1 创建FAST Cache5.2 管理FAST Cache传统的Raid Group只能针对某一个LUN进行设置：Storage Pool 则是在Pool级别进行设置：。