One way to guard against out-of-memory errors in applications is to add some swap space to your server. In this guide, we will cover how to add a swap file to an Debian 11 server.<\/p>\n
Swap<\/em>\u00a0is a portion of hard drive storage that has been set aside for the operating system to temporarily store data that it can no longer hold in RAM. This lets you increase the amount of information that your server can keep in its working memory, with some caveats. The swap space on the hard drive will be used mainly when there is no longer sufficient space in RAM to hold in-use application data.<\/p>\n The information written to disk will be significantly slower than information kept in RAM, but the operating system will prefer to keep running application data in memory and use swap for the older data. Overall, having swap space as a fallback for when your system\u2019s RAM is depleted can be a good safety net against out-of-memory exceptions on systems with non-SSD storage available.<\/p>\n Before we begin, we can check if the system already has some swap space available. It is possible to have multiple swap files or swap partitions, but generally one should be enough.<\/p>\n We can see if the system has any configured swap by typing:<\/p>\n Copy<\/p>\n If you don\u2019t get back any output, this means your system does not have swap space available currently.<\/p>\n You can verify that there is no active swap using the\u00a0 Copy<\/p>\n As you can see in the\u00a0Swap<\/strong>\u00a0row of the output, no swap is active on the system.<\/p>\n Before we create our swap file, we\u2019ll check our current disk usage to make sure we have enough space. Do this by entering:<\/p>\n Copy<\/p>\n The device with\u00a0 Although there are many opinions about the appropriate size of a swap space, it really depends on your personal preferences and your application requirements. Generally, an amount equal to or double the amount of RAM on your system is a good starting point. Another good rule of thumb is that anything over 4G of swap is probably unnecessary if you are just using it as a RAM fallback.<\/p>\n Now that we know our available hard drive space, we can create a swap file on our filesystem. We will allocate a file of the size that we want called\u00a0 The best way of creating a swap file is with the\u00a0 Since the server in our example has 1G of RAM, we will create a 1G file in this guide. Adjust this to meet the needs of your own server:<\/p>\n Copy<\/p>\n We can verify that the correct amount of space was reserved by typing:<\/p>\n Copy<\/p>\n Copy<\/p>\n Our file has been created with the correct amount of space set aside.<\/p>\n Now that we have a file of the correct size available, we need to actually turn this into swap space.<\/p>\n First, we need to lock down the permissions of the file so that only users with\u00a0root<\/strong>\u00a0privileges can read the contents. This prevents normal users from being able to access the file, which would have significant security implications.<\/p>\n Make the file only accessible to\u00a0root<\/strong>\u00a0by typing:<\/p>\n Copy<\/p>\n Verify the permissions change by typing:<\/p>\n Copy<\/p>\n As you can see, only the\u00a0root<\/strong>\u00a0user has the read and write flags enabled.<\/p>\n We can now mark the file as swap space by typing:<\/p>\n Copy<\/p>\n After marking the file, we can enable the swap file, allowing our system to start using it:<\/p>\n Copy<\/p>\n Verify that the swap is available by typing:<\/p>\n Copy<\/p>\n We can check the output of the\u00a0 Copy<\/p>\n Our swap has been set up successfully and our operating system will begin to use it as necessary.<\/p>\n Our recent changes have enabled the swap file for the current session. However, if we reboot, the server will not retain the swap settings automatically. We can change this by adding the swap file to our\u00a0 Back up the\u00a0 Copy<\/p>\n Add the swap file information to the end of your\u00a0 Copy<\/p>\n Next we\u2019ll review some settings we can update to tune our swap space.<\/p>\n There are a few options that you can configure that will have an impact on your system\u2019s performance when dealing with swap.<\/p>\n The\u00a0 With values close to zero, the kernel will not swap data to the disk unless absolutely necessary. Remember, interactions with the swap file are \u201cexpensive\u201d 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.<\/p>\n 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\u2019 memory profile or what you are using your server for, this might be better in some cases.<\/p>\n We can see the current swappiness value by typing:<\/p>\n Copy<\/p>\n For a Desktop, a swappiness setting of 60 is not a bad value. For a server, you might want to move it closer to 0.<\/p>\n We can set the swappiness to a different value by using the\u00a0 For instance, to set the swappiness to 10, we could type:<\/p>\n Copy<\/p>\n This setting will persist until the next reboot. We can set this value automatically at restart by adding the line to our\u00a0 Copy<\/p>\n At the bottom, you can add:<\/p>\n \/etc\/sysctl.conf<\/p>\n Save and close the file when you are finished.<\/p>\n Another related value that you might want to modify is the\u00a0 Basically, this is access data about the filesystem. This is generally very costly to look up and very frequently requested, so it\u2019s an excellent thing for your system to cache. You can see the current value by querying the\u00a0 Copy<\/p>\n 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:<\/p>\n Copy<\/p>\n Again, this is only valid for our current session. We can change that by adding it to our configuration file like we did with our swappiness setting:<\/p>\n Copy<\/p>\n At the bottom, add the line that specifies your new value:<\/p>\n \/etc\/sysctl.conf<\/p>\n Save and close the file when you are finished.<\/p>\n Following the steps in this guide will give you some breathing room in cases that would otherwise lead to out-of-memory exceptions. Swap space can be incredibly useful in avoiding some of these common problems.<\/p>\n If you are running into out-of-memory errors, or if you find that your system is unable to use the applications you need, the best solution is to optimize your application configurations or upgrade your server.<\/p>\n","protected":false},"excerpt":{"rendered":" Introduction One way to guard against out-of-memory errors in applications is to add some swap space to your server. In this guide, we will cover how …<\/p>\nStep 1 \u2013 Checking the System for Swap Information<\/h2>\n
sudo swapon --show\r\n<\/code><\/pre>\nfree<\/code>\u00a0utility:<\/p>\nfree -h\r\n<\/code><\/pre>\nOutput total used free shared buff\/cache available\r\nMem: 976Mi 75Mi 623Mi 0.0Ki 276Mi 765Mi\r\nSwap: 0B 0B 0B<\/mark>\r\n<\/code><\/pre>\nStep 2 \u2013 Checking Available Space on the Hard Drive Partition<\/h2>\n
df -h\r\n<\/code><\/pre>\nOutputFilesystem Size Used Avail Use% Mounted on\r\nudev 472M 0 472M 0% \/dev\r\ntmpfs 98M 500K 98M 1% \/run\r\n\/dev\/vda1 25G 1.1G 23G 5% \/<\/mark>\r\ntmpfs 489M 0 489M 0% \/dev\/shm\r\ntmpfs 5.0M 0 5.0M 0% \/run\/lock\r\n\/dev\/vda15 124M 5.9M 118M 5% \/boot\/efi\r\ntmpfs 98M 0 98M 0% \/run\/user\/0\r\n<\/code><\/pre>\n\/<\/code>\u00a0in the\u00a0Mounted on<\/code>\u00a0column is our disk in this case. We have plenty of space available in this example (only 1.4G used). Your usage will probably be different.<\/p>\nStep 3 \u2013 Creating a Swap File<\/h2>\n
swapfile<\/code>\u00a0in our root (\/<\/code>) directory.<\/p>\nfallocate<\/code>\u00a0program. This command instantly creates a file of the specified size.<\/p>\nsudo fallocate -l 1G<\/mark> \/swapfile\r\n<\/code><\/pre>\nls -lh \/swapfile\r\n<\/code><\/pre>\n-rw-r--r-- 1 root root 1.0G Aug 23 11:14 \/swapfile\r\n<\/code><\/pre>\nStep 4 \u2013 Enabling the Swap File<\/h2>\n
sudo chmod 600 \/swapfile\r\n<\/code><\/pre>\nls -lh \/swapfile\r\n<\/code><\/pre>\nOutput-rw-------<\/mark> 1 root root 1.0G Aug 23 11:14 \/swapfile\r\n<\/code><\/pre>\nsudo mkswap \/swapfile\r\n<\/code><\/pre>\nOutputSetting up swapspace version 1, size = 1024 MiB (1073737728 bytes)\r\nno label, UUID=6e965805-2ab9-450f-aed6-577e74089dbf\r\n<\/code><\/pre>\nsudo swapon \/swapfile\r\n<\/code><\/pre>\nsudo swapon --show\r\n<\/code><\/pre>\nOutputNAME TYPE SIZE USED PRIO\r\n\/swapfile file 1024M 0B -2\r\n<\/code><\/pre>\nfree<\/code>\u00a0utility again to corroborate our findings:<\/p>\nfree -h\r\n<\/code><\/pre>\nOutput total used free shared buff\/cache available\r\nMem: 976Mi 85Mi 612Mi 0.0Ki 279Mi 756Mi\r\nSwap: 1.0Gi 0B 1.0Gi<\/mark>\r\n<\/code><\/pre>\nStep 5 \u2013 Making the Swap File Permanent<\/h2>\n
\/etc\/fstab<\/code>\u00a0file.<\/p>\n\/etc\/fstab<\/code>\u00a0file in case anything goes wrong:<\/p>\nsudo cp \/etc\/fstab \/etc\/fstab.bak\r\n<\/code><\/pre>\n\/etc\/fstab<\/code>\u00a0file by typing:<\/p>\necho '\/swapfile none swap sw 0 0' | sudo tee -a \/etc\/fstab\r\n<\/code><\/pre>\nStep 6 \u2013 Tuning your Swap Settings<\/h2>\n
Adjusting the Swappiness Property<\/h3>\n
swappiness<\/code>\u00a0parameter 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.<\/p>\ncat \/proc\/sys\/vm\/swappiness\r\n<\/code><\/pre>\nOutput60\r\n<\/code><\/pre>\nsysctl<\/code>\u00a0command.<\/p>\nsudo sysctl vm.swappiness=10\r\n<\/code><\/pre>\nOutputvm.swappiness = 10\r\n<\/code><\/pre>\n\/etc\/sysctl.conf<\/code>\u00a0file:<\/p>\nsudo nano \/etc\/sysctl.conf\r\n<\/code><\/pre>\nvm.swappiness=10\r\n<\/code><\/pre>\nAdjusting the Cache Pressure Setting<\/h3>\n
vfs_cache_pressure<\/code>. This setting configures how much the system will choose to cache\u00a0inode<\/em>\u00a0and\u00a0dentry<\/em>\u00a0information over other data.<\/p>\nproc<\/code>\u00a0filesystem again:<\/p>\ncat \/proc\/sys\/vm\/vfs_cache_pressure\r\n<\/code><\/pre>\nOutput100\r\n<\/code><\/pre>\nsudo sysctl vm.vfs_cache_pressure=50\r\n<\/code><\/pre>\nOutputvm.vfs_cache_pressure = 50\r\n<\/code><\/pre>\nsudo nano \/etc\/sysctl.conf\r\n<\/code><\/pre>\nvm.vfs_cache_pressure=50\r\n<\/code><\/pre>\nConclusion<\/h2>\n