How to clear the filesystem buffer cache in Linux

You would probably want to do this if you’re suspecting that there’s an issue where you’re falling short on memory, diagnosing performance issues, etc. It’s much faster than a reboot and if it solves your problem, you’ll know what to do in the future. It’s not likely that you’ll need to do this. Just good to know how.

If you’re interested in more information regarding your memory, you can go look at: https://www.linuxatemyram.com/

Taken from: https://stackoverflow.com/questions/29870068/what-are-pagecache-dentries-inodes

With some oversimplification, let me try to explain in what appears to be the context of your question because there are multiple answers.

It appears you are working with memory caching of directory structures. An inode in your context is a data structure that represents a file. A dentries is a data structure that represents a directory. These structures could be used to build a memory cache that represents the file structure on a disk. To get a directly listing, the OS could go to the dentries–if the directory is there–list its contents (a series of inodes). If not there, go to the disk and read it into memory so that it can be used again.

The page cache could contain any memory mappings to blocks on disk. That could conceivably be buffered I/O, memory mapped files, paged areas of executables–anything that the OS could hold in memory from a file.

The commands flush these buffers.

Every Linux System has three options to clear cache without interrupting any processes or services.

  1. Clear PageCache only.
    • # sync; echo 1 > /proc/sys/vm/drop_caches
  2. Clear dentries and inodes.
    • # sync; echo 2 > /proc/sys/vm/drop_caches
  3. Clear PageCache, dentries and inodes.
    • # sync; echo 3 > /proc/sys/vm/drop_caches

Explanation of above commands. sync will flush the file system buffer. Commands separated by “;” run sequentially. The shell wait for each command to terminate before executing the next command in the sequence. As mentioned in kernel documentation, writing to drop_cache will clean cache without killing any application/service, command echo is doing the job of writing to file. If you have to clear the disk cache, the first command is safest in enterprise and production as “…echo 1 > ….” will clear the PageCache only. It is not recommended to use third option above “…echo 3 >” in production until you know what you are doing, as it will clear PageCache, dentries and inodes. Is it a good idea to free Buffer and Cache in Linux that might be used by Linux Kernel? When you are applying various settings and want to check, if it is actually implemented specially on I/O-extensive benchmark, then you may need to clear buffer cache. You can drop cache as explained above without rebooting the System i.e., no downtime required. Linux is designed in such a way that it looks into disk cache before looking onto the disk. If it finds the resource in the cache, then the request doesn’t reach the disk. If we clean the cache, the disk cache will be less useful as the OS will look for the resource on the disk. Moreover it will also slow the system for a few seconds while the cache is cleaned and every resource required by OS is loaded again in the disk-cache. Now we will be creating a shell script to auto clear RAM cache daily at 2am via a cron scheduler task. Create a shell script clearcache.sh and add the following lines. #!/bin/bash # Note, we are using “echo 3”, but it is not recommended in production instead use “echo 1” echo “echo 3 > /proc/sys/vm/drop_caches” Set execute permission on the clearcache.sh file. # chmod 755 clearcache.sh Now you may call the script whenever you required to clear ram cache. Now set a cron to clear RAM cache everyday at 2am. Open crontab for editing. # crontab -e Append the below line, save and exit to run it at 2am daily. 0 2 * * * /path/to/clearcache.sh For more details on how to cron a job you may like to check our article on 11 Cron Scheduling Jobs. Is it good idea to auto clear RAM cache on production server? No! it is not. Think of a situation when you have scheduled the script to clear ram cache everyday at 2am. Everyday at 2am the script is executed and it flushes your RAM cache. One day for whatsoever reason, may be more than expected users are online on your website and seeking resource from your server. At the same time scheduled script run and clears everything in cache. Now all the user are fetching data from disk. It will result in server crash and corrupt the database. So clear ram-cache only when required,and known your foot steps, else you are a Cargo Cult System Administrator. How to Clear Swap Space in Linux? If you want to clear Swap space, you may like to run the below command. # swapoff -a && swapon -a Also you may add above command to a cron script above, after understanding all the associated risk. Now we will be combining both above commands into one single command to make a proper script to clear RAM Cache and Swap Space. # echo 3 > /proc/sys/vm/drop_caches && swapoff -a && swapon -a && printf ‘\n%s\n’ ‘Ram-cache and Swap Cleared’ OR $ su -c “echo 3 >’/proc/sys/vm/drop_caches’ && swapoff -a && swapon -a && printf ‘\n%s\n’ ‘Ram-cache and Swap Cleared'” root After testing both above command, we will run command “free -h” before and after running the script and will check cache. Clear RAM Cache and Swap Space That’s all for now, if you liked the article, don’t forget to provide us with your valuable feedback in the comments to let us know, what you think is it a good idea to clear ram cache and buffer in production and Enterprise? Sharing is Caring…

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