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清除linux (ubuntu)下的缓存方法

2017-10-31

清除linux (ubuntu)下的缓存方法。Emptying the buffers cache,If you ever want to empty it you can use this chain of commands

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     cached
Mem:       1018916     980832      38084          0      46924     355764
-/+ buffers/cache:     578144     440772
Swap:      2064376        128    2064248
             total       used       free     shared    buffers     cached
Mem:       1018916     685008     333908          0        224     108252
-/+ buffers/cache:     576532     442384
Swap:      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     cached
    Mem:       7987492    7298164     689328          0      30416     457936
    -/+ buffers/cache:    6809812    1177680
    Swap:      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     cached
    Mem:       7987492    7777912     209580          0      39332     489864
    -/+ buffers/cache:    7248716     738776
    Swap:            0          0          0
    

    And to re-enable it:

    $ swapon -a
    

    And now reconfirm with free:

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