如何清空buffers and cache in linux system

轉載自http://unix.stackexchange.com/questions/87908/how-do-you-empty-the-buffers-and-cache-on-a-linux-system

 

Emptying the buffers cache

If you ever want to empty it you can use this chain of commands.

$ free && sync && echo 3 > /proc/sys/vm/drop_caches && free             total       used       free     shared    buffers     cachedMem:       1018916     980832      38084          0      46924     355764-/+ buffers/cache:     578144     440772Swap:      2064376        128    2064248             total       used       free     shared    buffers     cachedMem:       1018916     685008     333908          0        224     108252-/+ buffers/cache:     576532     442384Swap:      2064376        128    2064248

You can signal the Linux Kernel to drop various aspects of cached items by changing the numeric argument to the above command.

• To free pagecache:

  $ echo 1 > /proc/sys/vm/drop_caches

• To free dentries and inodes:

  $ echo 2 > /proc/sys/vm/drop_caches

• To free pagecache, dentries and inodes:

  $ echo 3 > /proc/sys/vm/drop_caches

The above are meant to be run as root. If you're trying to do them using sudo then you'll need to change the syntax slightly to something like these:

$ sudo sh -c 'echo 1 >/proc/sys/vm/drop_caches'$ sudo sh -c 'echo 2 >/proc/sys/vm/drop_caches'$ sudo sh -c 'echo 3 >/proc/sys/vm/drop_caches'

NOTE: There's a more esoteric version of the above command if you're into that:

$ echo "echo 1 > /proc/sys/vm/drop_caches" | sudo sh

Why the change in syntax? The /bin/echo program is running as root, because of sudo, but the shell that's redirecting echo's output to the root-only file is still running as you. Your current shell does the redirection before sudo starts.

Seeing what's in the buffers and cache

Take a look at linux-ftools if you'd like to analyze the contents of the buffers & cache. Specifically if you'd like to see what files are currently being cached.

fincore

With this tool you can see what files are being cached within a give directory.

fincore [options] files...  --pages=false      Do not print pages  --summarize        When comparing multiple files, print a summary report  --only-cached      Only print stats for files that are actually in cache.

For example, /var/lib/mysql/blogindex:

root@xxxxxx:/var/lib/mysql/blogindex# fincore --pages=false --summarize --only-cached * stats for CLUSTER_LOG_2010_05_21.MYI: file size=93840384 , total pages=22910 , cached pages=1 , cached size=4096, cached perc=0.004365 stats for CLUSTER_LOG_2010_05_22.MYI: file size=417792 , total pages=102 , cached pages=1 , cached size=4096, cached perc=0.980392 stats for CLUSTER_LOG_2010_05_23.MYI: file size=826368 , total pages=201 , cached pages=1 , cached size=4096, cached perc=0.497512 stats for CLUSTER_LOG_2010_05_24.MYI: file size=192512 , total pages=47 , cached pages=1 , cached size=4096, cached perc=2.127660 stats for CLUSTER_LOG_2010_06_03.MYI: file size=345088 , total pages=84 , cached pages=43 , cached size=176128, cached perc=51.190476 stats for CLUSTER_LOG_2010_06_04.MYD: file size=1478552 , total pages=360 , cached pages=97 , cached size=397312, cached perc=26.944444 stats for CLUSTER_LOG_2010_06_04.MYI: file size=205824 , total pages=50 , cached pages=29 , cached size=118784, cached perc=58.000000 stats for COMMENT_CONTENT_2010_06_03.MYI: file size=100051968 , total pages=24426 , cached pages=10253 , cached size=41996288, cached perc=41.975764 stats for COMMENT_CONTENT_2010_06_04.MYD: file size=716369644 , total pages=174894 , cached pages=79821 , cached size=326946816, cached perc=45.639645 stats for COMMENT_CONTENT_2010_06_04.MYI: file size=56832000 , total pages=13875 , cached pages=5365 , cached size=21975040, cached perc=38.666667 stats for FEED_CONTENT_2010_06_03.MYI: file size=1001518080 , total pages=244511 , cached pages=98975 , cached size=405401600, cached perc=40.478751 stats for FEED_CONTENT_2010_06_04.MYD: file size=9206385684 , total pages=2247652 , cached pages=1018661 , cached size=4172435456, cached perc=45.321117 stats for FEED_CONTENT_2010_06_04.MYI: file size=638005248 , total pages=155763 , cached pages=52912 , cached size=216727552, cached perc=33.969556 stats for FEED_CONTENT_2010_06_04.frm: file size=9840 , total pages=2 , cached pages=3 , cached size=12288, cached perc=150.000000 stats for PERMALINK_CONTENT_2010_06_03.MYI: file size=1035290624 , total pages=252756 , cached pages=108563 , cached size=444674048, cached perc=42.951700 stats for PERMALINK_CONTENT_2010_06_04.MYD: file size=55619712720 , total pages=13579031 , cached pages=6590322 , cached size=26993958912, cached perc=48.533080 stats for PERMALINK_CONTENT_2010_06_04.MYI: file size=659397632 , total pages=160985 , cached pages=54304 , cached size=222429184, cached perc=33.732335 stats for PERMALINK_CONTENT_2010_06_04.frm: file size=10156 , total pages=2 , cached pages=3 , cached size=12288, cached perc=150.000000 ---total cached size: 32847278080

With the above output you can see that there are several *.MYD, *.MYI, and *.frm files that are currently being cached.

Swap

If you want to clear out your swap you can use the following commands.

$ free             total       used       free     shared    buffers     cachedMem:       7987492    7298164     689328          0      30416     457936-/+ buffers/cache:    6809812    1177680Swap:      5963772     609452    5354320

Then use this command to disable swap:

$ swapoff -a

You can confirm that it's now empty:

$ free             total       used       free     shared    buffers     cachedMem:       7987492    7777912     209580          0      39332     489864-/+ buffers/cache:    7248716     738776Swap:            0          0          0

And to re-enable it:

$ swapon -a

And now reconfirm with free:

$ free             total       used       free     shared    buffers     cachedMem:       7987492    7785572     201920          0      41556     491508-/+ buffers/cache:    7252508     734984Swap:      5963772          0    5963772