How to Flush Memory Cache and Buffer Cache on Linux Server

When you run the "free -m" command and get the below output, then you observe free memory section will be low value but comparatively buffers+cache value would be higher.

Now this is not a bad thing actually since your OS has reserved this memory to speed up your most used process by keeping them in the cache. But in case any new process is executed and your system is low on memory then these cache would be automatically released to make space for memory reservation of new processes.

[root@localhost]# free -m
             total       used       free     shared    buffers     cached
Mem:          2345       1234        1145          0         24        400
-/+ buffers/cache:        664        345
Swap:         4032          0       4032

if you want to clear or free cache/buffer memory then you need to run the below command.

[root@localhost]# echo 3 > /proc/sys/vm/drop_caches

These are the different values which you can use with the above command

echo 1 is clearing only page cache

[root@localhost]# echo 1 > /proc/sys/vm/drop_caches

echo 2 is to clear free dentries and inodes

[root@localhost]# echo 2 > /proc/sys/vm/drop_caches

echo 3 is clearing page cache, dentries and inodes

[root@localhost]# echo 3 > /proc/sys/vm/drop_caches

SSH login without password in linux

If you want to connect one Linux host to other Linux host through SSH with password-less connection then you need to perform below steps.

Lets suppose you need password-less login from host "server01" / user "redhat" to host "server02" / user "centos".

1. First login in on "server01"as user "redhat" and generate a pair of authentication keys.

[redhat@server01]# ssh-keygen -t rsa

Generating public/private rsa key pair.
Enter file in which to save the key (/home/redhat/.ssh/id_rsa):
Created directory '/home/redhat/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/redhat/.ssh/id_rsa.
Your public key has been saved in /home/redhat/.ssh/id_rsa.pub.
The key fingerprint is:
1e:4f:05:79:3a:9f:96:7c:2b:ad:e9:58:37:sc:37:e4 redhat@server01

Note: Do not enter a passphrase.

2. Now you use ssh to create a directory ~/.ssh as user "centos" on server02.

Note: If directory already exist, you do not create it again.

[redhat@server01]# ssh centos@server02 mkdir -p .ssh

centos@server02's password:

Finally append redhat's new public key to centos@server02:.ssh/authorized_keys and enter centos's password one last time:

3. Now copy the rsa key to server 02 ssh authorized_keys file

[redhat@server01]# cat .ssh/id_rsa.pub | ssh centos@server02 'cat >> .ssh/authorized_keys'

centos@server02's password:

Now you can log into server02 as "centos" from server01 as "redhat"a without password.

4. Now you can test the password less connection.

[redhat@server01]# ssh centos@server02

You are successfully login on the server02 without any password.

Note:  In case of any permission issue you need to set "700" permission on .ssh folder on server02.