[转载]How To Add Swap on Ubuntu 12.04
How To Add Swap on Ubuntu 12.04
About Linux Swapping
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 256MB— 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=256k
“of=/swapfile” designates the file’s name. In this case the name is swapfile.
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
