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

yum install flashcache-utils kmod-flashcache

配置:

用sdc 做sdb的缓存

flashcache_create -p back  cachedev /dev/sdc /dev/sdb

cachedev cachedev, ssd_devname /dev/sdc, disk_devname /dev/sdb cache mode WRITE_BACK

block_size 8, md_block_size 8, cache_size 0

Flashcache metadata will use 1192MB of your 24016MB main memory

可以用3种方式：

Writethrough - safest, all writes are cached to ssd but also written to disk

immediately.  If your ssd has slower write performance than your disk (likely

for early generation SSDs purchased in 2008-2010), this may limit your system

write performance.  All disk reads are cached (tunable).

数据同时写到ssd和普通硬盘

Writearound - again, very safe, writes are not written to ssd but directly to

disk.  Disk blocks will only be cached after they are read.  All disk reads

are cached (tunable).

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

based on various policies are written to disk later.  All disk reads are

cached (tunable).

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

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

fdisk -l /dev/mapper/cachedev  可以看到一个设备

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

pvcreate /dev/mapper/cachedev

Physical volume "/dev/mapper/cachedev" successfully created

vgcreate ssdcachetest /dev/mapper/cachedev

Volume group "ssdcachetest" successfully created

lvcreate -L 100G -n ssdcache_test_lv1 ssdcachetest

Logical volume "ssdcache_test_lv1" created

删除

dmsetup  remove cachedev

flashcache_destroy /dev/sdc