在Linux系统中将SSD当块设备缓存的方法
在Linux系统中将SSD当块设备缓存的方法
发布时间:2016-12-27 来源:查字典编辑
摘要: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...

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

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

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).

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).

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).

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

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

</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

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

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