在Linux系统中将SSD当块设备缓存的方法

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

复制代码

代码如下:

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

配置:

用sdc 做sdb的缓存

 

复制代码

代码如下:

</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

可以用3种方式：

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代码如下:

</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).

数据同时写到ssd和普通硬盘

 

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代码如下:

</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).

数据同时绕过ssd，直接写到普通硬盘

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代码如下:

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).

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([e-mail])

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 

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代码如下:

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

 

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代码如下:

</p> <p> pvcreate /dev/mapper/cachedev</p> <p> physical volume "/dev/mapper/cachedev" successfully created</p> <p> vgcreate ssdcachetest /dev/mapper/cachedev </p> <p> volume group "ssdcachetest" successfully created</p> <p> lvcreate -l 100g -n ssdcache_test_lv1 ssdcachetest</p> <p> logical volume "ssdcache_test_lv1" created

删除

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代码如下:

dmsetup remove cachedev</p> <p>flashcache_destroy /dev/sdc