Swap Space

Today i am going to talk about Swap Space, not going to explain (for now) how to make the Swap Space but i will talk about what it is.
Many people don't know what is Swap Space, but in fact it is an important thing in Linux, so let's go ahead with it.

If you don't know, Linux uses pagination to change memory blocks(pages) between the main memory (RAM) and hard drive, this way the programs that are not running at the moment can place parts of their memory on the hard drive, giving the new process's more memory to run. So it's in this step that Swap Space comes in action, Swap is the name of the space on the disk given to the Linux kernel to do the pagination. Normally the Swap size it's 2x your Physical Memory size, if you have 1GB RAM your Swap will be of 2GB, since disk space it's cheap this is no problem, the problem is as you may know RAM access is made in nanoseconds while disk access is done in milliseconds, if you have a big Swap size it will slow down your programs, because the overhead of pagination will be bigger.

Exists two ways of making a Swap Space the Swap Partition and the Swap File. Swap Partition it's the best way and the more used since it creates a partition on the disk dedicated to the Swap, while Swap File creates a file of the size you want your Swap size to have and it will consume disk space.

Swap size plus the Physical size it's called the Virtual Memory, this way you can run programs that use more memory than you have physically, without receive the message "Error allocating memory: out of memory".

In the new kernels there's a new configuration related to the Swap it's called Swappiness.
Swappiness can have a value of 0 to 100 and the default is 60, this consists in the times that kernel do the pagination, higher the number, more times kernel will do pagination, smaller the number, less times kernel will change memory pages to disk.

Conclusion

Swap Space is important on Linux and you always should use it as a partition.
You can use a Linux without Swap, but if your machine runs out of memory your system will crash.
The rule to choose the size of the Swap size it's 2x your physical memory size.


This was my first article and i am sorry about my english.
Hope you enjoy, if you have any comments or doubt leave a comment

How to: Disable IPTables

If you need to disable your iptables (not recommended) this is the way to do it.


Difficult: easy
Avg Time: 5min


You need to be logged as root to disable iptables.


Disable iptables on Ubuntu/Debian.

1º - Save your current iptables config, to restore later if needed.

iptables-save > /root/iptables.backup

2º -  Make this commands

iptables -X
iptables -t nat -F
iptables -t nat -X
iptables -t mangle -F
iptables -t mangle -X
iptables -P INPUT ACCEPT
iptables -P FORWARD ACCEPT
iptables -P OUTPUT ACCEPT

3º - Now if you would like to restore your old config, do this

iptables-restore < /root/iptables.backup

Disable iptables Centos/RedHat/Fedora.


Disable iptables on Centos/RedHat/Fedora it's much simple!

1º - Save current config and Stop

/etc/init.d/iptables save
/etc/init.d/iptables stop

2º - Start iptables again 

/etc/init.d/iptables start

If you have any problem leave a comment

How to: Add a new user

Hello, in my first post i will explain how to add a new user to your system.

Difficult: easy
Avg Time: 1 min


Linux have a built in command called "useradd" that help people to add new user to the system.
To add a new user you must be logged as root or you will need to use sudo.
I will give some of the must useful examples.

Syntax: useradd options name

Example 1: Add new user to the system

useradd -r ricardo

Example 2: Add new user and do not create home directory (/home)

useradd -rM ricardo

Example 3: Add new user and create home directory in /home

useradd -rm ricardo

Example 4: Add new user and specify the home directory

useradd -rd /somedir/somedir ricardo

Example 5: Add a user and create a group with the same name

useradd -rn /somedir/somedir ricardo

Example 6: Add a user and add user to existent group

useradd -r -g groupname ricardo

Example 7: Add a user, create home directory and a group with same name

useradd -rmn ricardo

If you would like to set a password for the user i recommend you to use the passwd command instead of the -p option of useradd command.

Example 8: Create new user and set the password

useradd -rm ricardo

passwd ricardo

That's the way i use to add new user and set user password, if you have any other suggestion please post it as comment