原理
写操作先缓存到ssd硬盘上，然后通过一定策略写到普通硬盘上；
读操作热点数据可以缓存到ssd硬盘上，提高读取数据的速度。
软件 ssd 缓存开源软件主要有bcache ，flashcache
bcache 需要编译最新的内核，要求比较高，配置比较复杂，目前主要用于测试环境；
flashcache 是facebook内部使用的ssd cache软件，开源出来供大家使用，目前有人编译出rhel6的rpm包，在centos6上可以直接安装使用。
本次初步测试了flashcache
测试环境
sdc为 ssd 硬盘 160G*4 raid 10 用fio测试 4k 写iops为 5433
sdb为 普通sas硬盘 146G*2 raid 1 用fio测试 4k 写iops为 447
初步简单测试结果：
4k 随机写可以达到 5014
4k随机读可以达到45874
flashcache 安装配置方法
安装
rpm --import http://elrepo.org/RPM-GPG-KEY-elrepo.org</p>
<p>rpm -Uvh http://elrepo.org/elrepo-release-6-5.el6.elrepo.noarch.rpm</p>
<p>yum install flashcache-utils kmod-flashcache</div>
配置:
用sdc 做sdb的缓存
</p>
<p>flashcache_create -p back cachedev /dev/sdc /dev/sdb </p>
<p>cachedev cachedev, ssd_devname /dev/sdc, disk_devname /dev/sdb cache mode WRITE_BACK</p>
<p>block_size 8, md_block_size 8, cache_size 0</p>
<p>Flashcache metadata will use 1192MB of your 24016MB main memory</div>
可以用3种方式：
</p>
<p>Writethrough - safest, all writes are cached to ssd but also written to disk</p>
<p>immediately. If your ssd has slower write performance than your disk (likely</p>
<p>for early generation SSDs purchased in 2008-2010), this may limit your system</p>
<p>write performance. All disk reads are cached (tunable). </div>
数据同时写到ssd和普通硬盘
</p>
<p>Writearound - again, very safe, writes are not written to ssd but directly to</p>
<p>disk. Disk blocks will only be cached after they are read. All disk reads</p>
<p>are cached (tunable).</div>
数据同时绕过ssd，直接写到普通硬盘
Writeback - fastest but less safe. Writes only go to the ssd initially, and</p>
<p>based on various policies are written to disk later. All disk reads are</p>
<p>cached (tunable). </div>
数据先写到ssd，随后写到普通硬盘
查看
flashstat
======================================================================================================
Flashstat: a tool for flashcache status per second
Author : NinGoo(seaman.ning@gmail.com)
Version : 0.3
======================================================================================================
SSD Device: /dev/sdc Disk Device: /dev/sdb Cache Mode: WRITE_BACK
Capacity: 303998M Block Size: 4K Meta Block Size: 4096b
Total Blocks: 77823488 Cached Blocks: 9 Cached Percent: 0
Set Numbers: 512 Dirty Blocks: 0 Dirty Percent: 0
cache_all: 1 reclaim_policy: FIFO dirty_thresh_pct: 20
max_clean_ios_set: 2 max_clean_ios_total: 4 skip_seq_thresh: 0K
======================================================================================================
time read/s write/s diskr/s diskw/s ssdr/s ssdw/s uread/s uwrit/s metaw/s clean/s repl/s wrepl/s hit% whit% dwhit%
03-13 14:51:20 0 0 0 0 0 0 0 0 0 0 0 0 0|10 0|0 0|0
Disk /dev/mapper/cachedev: 146.8 GB, 146778685440 bytes
255 heads, 63 sectors/track, 17844 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0002da0a
Device Boot Start End Blocks Id System
使用：
可以当一个块设备来使用，划分vg lv
</p>
<p> pvcreate /dev/mapper/cache