这篇文章主要介绍了在Linux系统中将SSD当块设备缓存的方法,主要是将SSD作为内存和HDD中间的过度缓存设备,需要的朋友可以参考下
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 -Uvh http://elrepo.org/elrepo-release-6-5.el6.elrepo.noarch.rpm
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
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
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
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
flashcache_destroy /dev/sdc
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