Linux RAM is composed of chunks of memory called pages. To free up pages of RAM, a “linux swap” can occur and a page of memory is copied from the RAM to preconfigured space on the hard disk. Linux swaps allow a system to harness more memory than was originally physically available.

However, swapping does have disadvantages. Because hard disks have a much slower memory than RAM, virtual private server performance may slow down considerably. Additionally, swap thrashing can begin to take place if the system gets swamped from too many files being swapped in and out.
Check for Swap Space
Before we proceed to set up a swap file, we need to check if any swap files have been enabled on the VPS by looking at the summary of swap usage.

sudo swapon -s

An empty list will confirm that you have no swap files enabled:

Filename                Type        Size    Used    Priority

Check the File System
After we know that we do not have a swap file enabled on the virtual server, we can check how much space we have on the server with the df command. The swap file will take 512MB— since we are only using up about 8% of the /dev/sda, we can proceed.

df
Filesystem     1K-blocks    Used Available Use% Mounted on
/dev/sda        20907056 1437188  18421292   8% /
udev              121588       4    121584   1% /dev
tmpfs              49752     208     49544   1% /run
none                5120       0      5120   0% /run/lock
none              124372       0    124372   0% /run/shm

Create and Enable the Swap File
Now it’s time to create the swap file itself using the dd command :

sudo dd if=/dev/zero of=/swapfile bs=1024 count=2048k

“of=/swapfile” designates the file’s name. In this case the name is swapfile.

—————————————————————————————————————————–
Sizing:
bs=1024:  Each block is made of 1024 bytes
count=2048K: There will be 2048K blocks, so
(bs x  count) = (1024 x 2,048,000)
This would create a 2 GB swapfile.
—————————————————————————————————————————–

You can change count to 1024K if you want 1 Gigabyte of swap, more than that is not really recommended.

Subsequently we are going to prepare the swap file by creating a linux swap area:

sudo mkswap /swapfile

The results display:

Setting up swapspace version 1, size = 262140 KiB
no label, UUID=103c4545-5fc5-47f3-a8b3-dfbdb64fd7eb

Finish up by activating the swap file:

sudo swapon /swapfile

You will then be able to see the new swap file when you view the swap summary.

swapon -s
Filename                Type        Size    Used    Priority
/swapfile                               file        262140    0    -1

This file will last on the virtual private server until the machine reboots. You can ensure that the swap is permanent by adding it to the fstab file.

Open up the file:

sudo nano /etc/fstab

Paste in the following line:

/swapfile       none    swap    sw      0       0

Swappiness in the file should be set to 10. Skipping this step may cause both poor performance, whereas setting it to 10 will cause swap to act as an emergency buffer, preventing out-of-memory crashes.

You can do this with the following commands:

echo 10 | sudo tee /proc/sys/vm/swappiness
echo vm.swappiness = 10 | sudo tee -a /etc/sysctl.conf

To prevent the file from being world-readable, you should set up the correct permissions on the swap file:

sudo chown root:root /swapfile
sudo chmod 0600 /swapfile

By: Digital Ocean

Cron can be touchy when doing multiple operations.
I wanted to use rsync over ssh while passing a password using expect.
The best way to do this is to create a script file and then point cron to the script.
One caveat I ran into is that a timeout needs to be set.  This setting needs to be long enough that the files are copied before the timeout occurs.Otherwise you do not copy all of the files.

Here is an example:

#!/usr/bin/expect -f
set timeout 86400
spawn /usr/bin/rsync -e ssh -av user@remotelocation:/remotedirectorylocation /localdirectorylocation
expect {
“*Password:*”
{send “userpasswordr”
}
}
expect eof
exit

Make a mount point:

mkdir /mnt/share

Then, mount your smb share:

mount.cifs //192.168.0.6/sharename /mnt/share -o user=username

It’ll prompt you for a password (you want to stay away from typing passwords within commands when you can!)

Verify it’s mounted by using the mount command:

mount
//192.168.0.6/sharename on /mnt/share type cifs (rw)

Want to do it automagically at every boot? Add it to /etc/fstab:

//192.168.0.6/share    /mnt/share        smbfs    username=rob,password=SuPeRdUpEr 0 0

Now you can rsync stuff to it – let’s rsync rob’s home directory into a dir called ‘homedir’:

rsync -avz /home/rob/ /mnt/share/homedir/

Now, if you really want to get fancy and feel all backed up all the time, add an rsync to crontab!

crontab -e

And add your rsync line to go every night at 2am (or whenever) w/o emailing root anything.

* 2 * * * /usr/bin/rsync -avz /home/rob/ /mnt/share/homedir/ >/dev/null 2>&1

One thing to make sure of though – ensure that your NAS will always come up under that same ip address or your system won’t be able to mount it – and you’ll be rsyncing your home directory into your /mnt/share/homedir on your local system.

By: Rob

Using ssh keys is always one of the safest ways to connect rsync through ssh.
Here is an alternate less secure way to use rsync when in a pinch.

Install expect

Run the following command:

expect -c ‘spawn rsync -e ssh -avz username@servernameorip:/pathonremoteserver/directory /localdirectory; expect “*?assword:*” {send “userpasswordr”; interact};’