在Linux系統中將SSD當塊設備緩存的方法

2020-10-21 21:52:08

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原理

寫操作先緩存到ssd硬盤上，然后通過一定策略寫到普通硬盤上；

讀操作熱點數據可以緩存到ssd硬盤上，提高讀取數據的速度。

軟件 ssd 緩存開源軟件主要有bcache ，flashcache

bcache 需要編譯最新的內核，要求比較高，配置比較復雜，目前主要用于測試環境；

flashcache 是facebook內部使用的ssd cache軟件，開源出來供大家使用，目前有人編譯出rhel6的rpm包，在centos6上可以直接安裝使用。

本次初步測試了flashcache

測試環境

sdc為 ssd 硬盤 160G*4 raid 10 用fio測試 4k 寫iops為 5433

sdb為普通sas硬盤 146G*2 raid 1 用fio測試 4k 寫iops為 447

初步簡單測試結果：

4k 隨機寫可以達到 5014

4k隨機讀可以達到45874

flashcache 安裝配置方法

安裝

復制代碼

代碼如下:

rpm --import http://elrepo.org/RPM-GPG-KEY-elrepo.orgrpm -Uvh http://elrepo.org/elrepo-release-6-5.el6.elrepo.noarch.rpmyum install flashcache-utils kmod-flashcache

配置:

用sdc 做sdb的緩存

復制代碼

代碼如下:

flashcache_create -p back cachedev /dev/sdc /dev/sdb cachedev cachedev, ssd_devname /dev/sdc, disk_devname /dev/sdb cache mode WRITE_BACKblock_size 8, md_block_size 8, cache_size 0Flashcache metadata will use 1192MB of your 24016MB main memory

可以用3種方式：

復制代碼

代碼如下:

Writethrough - safest, all writes are cached to ssd but also written to diskimmediately. If your ssd has slower write performance than your disk (likelyfor early generation SSDs purchased in 2008-2010), this may limit your systemwrite performance. All disk reads are cached (tunable).

數據同時寫到ssd和普通硬盤

復制代碼

代碼如下:

Writearound - again, very safe, writes are not written to ssd but directly todisk. Disk blocks will only be cached after they are read. All disk readsare cached (tunable).

數據同時繞過ssd，直接寫到普通硬盤

復制代碼

代碼如下:

Writeback - fastest but less safe. Writes only go to the ssd initially, andbased on various policies are written to disk later. All disk reads arecached (tunable).

數據先寫到ssd，隨后寫到普通硬盤

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

復制代碼

代碼如下:

fdisk -l /dev/mapper/cachedev

可以看到一個設備

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：

可以當一個塊設備來使用，劃分vg lv

復制代碼

代碼如下:

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

刪除

復制代碼

代碼如下:

dmsetup remove cachedevflashcache_destroy /dev/sdc

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