Hack The Sec-Leading Resource Of Linux Tutorial: physical

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<u><span style="color: #cc0000;"><i>Add extra swap space on CentOS 6</i></span></u></h1>
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<span style="font-family: Georgia, Times New Roman, serif;">In <a href="http://www.hackthesec.co.in/search/label/Centos" target="_blank">CentOS 6</a> swap space or simply swap is something that is used by the operating system when the system requires more memory that the physical memory installed on the server. In that case, the operating system is using parts from the hard drives as if they were physical memory.</span></div>
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<span style="font-family: Georgia, Times New Roman, serif;">This sounds very good, but the only downside of this is that the hard drives are significantly slower than the RAM memory, so this will decrease the server performance in general. However, having enough memory available is very important for our server to be stable and run smooth so we should always have swap space available on our <a href="http://www.hackthesec.co.in/search/label/Linux" target="_blank">Linux server</a>.</span></div>
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<span style="font-family: Georgia, Times New Roman, serif;">Before we add more SWAP space, we need to create the swap file. We will use the <span style="color: red;"><b><code style="background-color: #f9f2f4; border-radius: 4px; box-sizing: border-box; font-size: 16.2px; padding: 2px 4px;">dd</code> </b></span>command to create the swap file. <span style="color: red;"><b><code style="background-color: #f9f2f4; border-radius: 4px; box-sizing: border-box; font-size: 16.2px; padding: 2px 4px;">dd</code> </b></span>is built in command in <a href="http://www.hackthesec.co.in/search/label/Centos" target="_blank">CentOS </a>whose primary purpose is to convert and copy a file .</span></div>
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<span style="font-family: Georgia, Times New Roman, serif;">In this case <span style="color: red;"><b><code style="background-color: #f9f2f4; border-radius: 4px; box-sizing: border-box; font-size: 16.2px; padding: 2px 4px;">if</code> </b></span>(input file) is <code style="background-color: #f9f2f4; border-radius: 4px; box-sizing: border-box; font-size: 16.2px; padding: 2px 4px;"><span style="color: red;"><b>/dev/zero</b></span></code> and <span style="color: red;"><b><code style="background-color: #f9f2f4; border-radius: 4px; box-sizing: border-box; font-size: 16.2px; padding: 2px 4px;">of</code> </b></span>(output file) is the destination where we want to save the file. Basically, we are going to create 1GB file that will consist of zeros only and use that as a swap later.</span></div>
<pre style="background-color: whitesmoke; border-radius: 4px; border: 1px solid rgb(204, 204, 204); box-sizing: border-box; color: #333333; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; line-height: 1.42857; margin-bottom: 10.5px; overflow: auto; padding: 10px; word-break: break-all; word-wrap: break-word;"><code style="background-color: transparent; border-radius: 0px; box-sizing: border-box; color: inherit; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; padding: 0px; white-space: pre-wrap;"><span style="font-size: large;"># dd if=/dev/zero of=/swapfile bs=1M count=1024
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<span style="font-family: Georgia, Times New Roman, serif;">Next, we need to set up file so it is can be used as swap space. We will use <span style="color: red;"><b><code style="background-color: #f9f2f4; border-radius: 4px; box-sizing: border-box; font-size: 16.2px; padding: 2px 4px;">mkswap</code> </b></span>command for that:</span></div>
<pre style="background-color: whitesmoke; border-radius: 4px; border: 1px solid rgb(204, 204, 204); box-sizing: border-box; color: #333333; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; line-height: 1.42857; margin-bottom: 10.5px; overflow: auto; padding: 10px; word-break: break-all; word-wrap: break-word;"><code style="background-color: transparent; border-radius: 0px; box-sizing: border-box; color: inherit; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; padding: 0px; white-space: pre-wrap;"><span style="font-size: large;"># mkswap /swapfile
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<span style="font-family: Georgia, Times New Roman, serif;">The swap file must be owned by root and have proper permissions:</span></div>
<pre style="background-color: whitesmoke; border-radius: 4px; border: 1px solid rgb(204, 204, 204); box-sizing: border-box; color: #333333; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; line-height: 1.42857; margin-bottom: 10.5px; overflow: auto; padding: 10px; word-break: break-all; word-wrap: break-word;"><code style="background-color: transparent; border-radius: 0px; box-sizing: border-box; color: inherit; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; padding: 0px; white-space: pre-wrap;"><span style="font-size: large;">#chown root:root /swapfile
#chmod 0600 /swapfile</span></code><code style="background-color: transparent; border-radius: 0px; box-sizing: border-box; color: inherit; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; font-size: inherit; padding: 0px; white-space: pre-wrap;">
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<span style="font-family: Georgia, Times New Roman, serif;">Once this is done, the only thing left is to activate and start using the swap space.</span></div>
<pre style="background-color: whitesmoke; border-radius: 4px; border: 1px solid rgb(204, 204, 204); box-sizing: border-box; color: #333333; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; line-height: 1.42857; margin-bottom: 10.5px; overflow: auto; padding: 10px; word-break: break-all; word-wrap: break-word;"><code style="background-color: transparent; border-radius: 0px; box-sizing: border-box; color: inherit; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; padding: 0px; white-space: pre-wrap;"><span style="font-size: large;"># swapon /swapfile
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<span style="font-family: Georgia, Times New Roman, serif;">That’s it. We got another 1GB of swap space. If we wont to keep the space active after system reboot, we need to add an entry into <code style="background-color: #f9f2f4; border-radius: 4px; box-sizing: border-box; font-size: 16.2px; padding: 2px 4px;"><span style="color: red;"><b>/etc/fstab</b></span></code> file. Open <code style="background-color: #f9f2f4; border-radius: 4px; box-sizing: border-box; font-size: 16.2px; padding: 2px 4px;"><span style="color: red;"><b>/etc/fstab</b></span></code> file using your favorite text editor and add the following entry:</span></div>
<pre style="background-color: whitesmoke; border-radius: 4px; border: 1px solid rgb(204, 204, 204); box-sizing: border-box; color: #333333; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; line-height: 1.42857; margin-bottom: 10.5px; overflow: auto; padding: 10px; word-break: break-all; word-wrap: break-word;"><code style="background-color: transparent; border-radius: 0px; box-sizing: border-box; color: inherit; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; padding: 0px; white-space: pre-wrap;"><span style="font-size: large;">/swapfile swap swap defaults 0 0
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<span style="font-family: Georgia, Times New Roman, serif;">Save and close the file. Next time Linux comes up after reboot, it enables the new swap file for you automatically.</span></div>
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<span style="font-family: Georgia, Times New Roman, serif;">We can verify that the swap was added by executing the command, the output should look like the following:</span></div>
<pre style="background-color: whitesmoke; border-radius: 4px; border: 1px solid rgb(204, 204, 204); box-sizing: border-box; color: #333333; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; line-height: 1.42857; margin-bottom: 10.5px; overflow: auto; padding: 10px; word-break: break-all; word-wrap: break-word;"><code style="background-color: transparent; border-radius: 0px; box-sizing: border-box; color: inherit; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; padding: 0px; white-space: pre-wrap;"><span style="font-size: large;"># free -m
             total       used       free     shared    buffers     cached
Mem:          2026        990       1036          0        129        620
-/+ buffers/cache:        240       1786
Swap:         1024          0       1024</span></code><code style="background-color: transparent; border-radius: 0px; box-sizing: border-box; color: inherit; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; font-size: inherit; padding: 0px; white-space: pre-wrap;">
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<span style="font-family: Georgia, Times New Roman, serif;">In order to determine how often swap file is used (compared to the RAM usage) system uses system property that is called swappiness. Swappiness is a property of the Linux kernel that changes the balance between swapping out runtime memory, as opposed to dropping pages from the system page cache. Swappiness can be set to values between 0 and 100 inclusive.</span></div>
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<span style="font-family: Georgia, Times New Roman, serif;">A low value means the kernel will try to avoid swapping as much as possible where a higher value instead will make the kernel aggressively try to use swap space. The default value is 60. We can set the swappiness with the following command:</span></div>
<pre style="background-color: whitesmoke; border-radius: 4px; border: 1px solid rgb(204, 204, 204); box-sizing: border-box; color: #333333; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; line-height: 1.42857; margin-bottom: 10.5px; overflow: auto; padding: 10px; word-break: break-all; word-wrap: break-word;"><code style="background-color: transparent; border-radius: 0px; box-sizing: border-box; color: inherit; font-family: Menlo, Monaco, Consolas, 'Courier New', monospace; padding: 0px; white-space: pre-wrap;"><span style="font-size: large;">sysctl vm.swappiness=10
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<span style="font-family: Georgia, Times New Roman, serif;">Now you have swap file on your system, which means you have more memory for the system to work with. This will make your system more stable at the cost of HDD space used by the swap file. </span></div>
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How to add extra swap space on CentOS 6.X

Add extra swap space on CentOS 6 In CentOS 6 swap space or simply swap is something that is used by the operating system when the system requires more memory that the physical memory installed on the…