Swap is 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.
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’s RAM is depleted can be a good safety net against out-of-memory exceptions on systems with non-SSD storage available.
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