Issue: เนื่องจากพบปัญหาว่าเครื่อง Server ที่เป็น VPS นั้นมี Memory(ram) น้อย ทำให้ Process บางตัวมันตายไปเองเช่น mysql server

การทำ swap memory หรือที่รู้จักกันในนาม Visual Memory เป็นสิ่งที่ตอบโจทย์อย่างยิ่งเนื่องจากมีการจัดการหน่วยความจำของเครื่องและหน่วยความจำเสมือน โดยมีการติดตั้งดังนี้

How To Add Swap on Ubuntu 14.04

The Faster Way

The quicker way of getting the same file is by using the fallocate program. This command creates a file of a preallocated size instantly, without actually having to write dummy contents.

We can create a 4 Gigabyte file by typing:

sudo fallocate -l 4G /swapfile

The prompt will be returned to you almost immediately. We can verify that the correct amount of space was reserved by typing:

ls -lh /swapfile

-rw-r--r-- 1 root root 4.0G Apr 28 17:19 /swapfile

As you can see, our file is created with the correct amount of space set aside.

Enabling the Swap File

Right now, our file is created, but our system does not know that this is supposed to be used for swap. We need to tell our system to format this file as swap and then enable it.

Before we do that though, we need to adjust the permissions on our file so that it isn’t readable by anyone besides root. Allowing other users to read or write to this file would be a huge security risk. We can lock down the permissions by typing:

sudo chmod 600 /swapfile

Verify that the file has the correct permissions by typing:

ls -lh /swapfile

-rw------- 1 root root 4.0G Apr 28 17:19 /swapfile

As you can see, only the columns for the root user have the read and write flags enabled.

Now that our file is more secure, we can tell our system to set up the swap space by typing:

sudo mkswap /swapfile

Setting up swapspace version 1, size = 4194300 KiB
no label, UUID=e2f1e9cf-c0a9-4ed4-b8ab-714b8a7d6944

Our file is now ready to be used as a swap space. We can enable this by typing:

sudo swapon /swapfile

We can verify that the procedure was successful by checking whether our system reports swap space now:

sudo swapon -s

Filename                Type        Size    Used    Priority
/swapfile               file        4194300 0       -1

We have a new swap file here. We can use the free utility again to corroborate our findings:

free -m

             total       used       free     shared    buffers     cached
Mem:          3953        101       3851          0          5         30
-/+ buffers/cache:         66       3887
Swap:         4095          0       4095

Our swap has been set up successfully and our operating system will begin to use it as necessary.

Make the Swap File Permanent

We have our swap file enabled, but when we reboot, the server will not automatically enable the file. We can change that though by modifying the fstab file.

Edit the file with root privileges in your text editor:

sudo nano /etc/fstab

At the bottom of the file, you need to add a line that will tell the operating system to automatically use the file you created:

/swapfile   none    swap    sw    0   0

Save and close the file when you are finished.

Tweak your Swap Settings

There are a few options that you can configure that will have an impact on your system’s performance when dealing with swap.

The swappiness parameter configures how often your system swaps data out of RAM to the swap space. This is a value between 0 and 100 that represents a percentage.

With values close to zero, the kernel will not swap data to the disk unless absolutely necessary. Remember, interactions with the swap file are “expensive” in that they take a lot longer than interactions with RAM and they can cause a significant reduction in performance. Telling the system not to rely on the swap much will generally make your system faster.

Values that are closer to 100 will try to put more data into swap in an effort to keep more RAM space free. Depending on your applications’ memory profile or what you are using your server for, this might be better in some cases.

We can see the current swappiness value by typing:

cat /proc/sys/vm/swappiness

For a Desktop, a swappiness setting of 60 is not a bad value. For a VPS system, we’d probably want to move it closer to 0.

We can set the swappiness to a different value by using the sysctl command.

For instance, to set the swappiness to 10, we could type:

sudo sysctl vm.swappiness=10

vm.swappiness = 10

This setting will persist until the next reboot. We can set this value automatically at restart by adding the line to our /etc/sysctl.conf file:

sudo nano /etc/sysctl.conf

At the bottom, you can add:

vm.swappiness=10

Save and close the file when you are finished.

Another related value that you might want to modify is the vfs_cache_pressure. This setting configures how much the system will choose to cache inode and dentry information over other data.

Basically, this is access data about the filesystem. This is generally very costly to look up and very frequently requested, so it’s an excellent thing for your system to cache. You can see the current value by querying the proc filesystem again:

cat /proc/sys/vm/vfs_cache_pressure

As it is currently configured, our system removes inode information from the cache too quickly. We can set this to a more conservative setting like 50 by typing: