**************************************************** * Description: Knowledge of Linux memory swap space * Date: 02:22 PM EST, 06/19/2017 **************************************************** <1> Linux Swap Space: | |__ o. Linux divides its physical RAM (random access memory) into chucks of memory called pages. | Swapping is the process whereby a page of memory is copied to the preconfigured space on the hard disk, called swap space, to free up that page of memory. | The combined sizes of the physical memory and the swap space is the amount of virtual memory available. | |__ o. Swapping is necessary for two important reasons. First, when the system requires more memory than is physically available, the kernel swaps out less used pages | and gives memory to the current application (process) that needs the memory immediately. Second, a significant number of the pages used by an application during | its startup phase may only be used for initialization and then never used again. The system can swap out those pages and free the memory for other applications | or even for the disk cache. | |__ o. However, swapping does have a downside. Compared to memory, disks are very slow. Memory speeds can be measured in nanoseconds, while disks are measured in milliseconds, | so accessing the disk can be tens of thousands times slower than accessing physical memory. The more swapping that occurs, the slower your system will be. | Sometimes excessive swapping or thrashing occurs where a page is swapped out and then very soon swapped in and then swapped out again and so on. | In such situations the system is struggling to find free memory and keep applications running at the same time. In this case only adding more RAM will help. | |__ o. Linux has two forms of swap space: the swap partition and the swap file. The swap partition is an independent section of the hard disk used solely for swapping; no other files can reside there. The swap file is a special file in the filesystem that resides amongst your system and data files. <2> Checking swap size: | |__ $ swapon -s Filename Type Size Used Priority /dev/dm-1 partition 14434300 48116 -1 ......... Swap space on disk. Used unit is KB. /mnt/resource/swapfile file 2097148 0 -1 ......... Swap space on file. Used unit is KB. <3> Creating swap file: | |__ $ su - ............................................................................................... All the below actions should be performed as root. | | |__ $ dd if=/dev/zero of=/mnt/resource/swapfile_1 bs=1024 count=5242880 .................................. To create a swap file. | Read operations from /dev/zero return as many null characters (0x00) | 5242880+0 records in as requested in the read operation. | 5242880+0 records out | 5368709120 bytes (5.4 GB) copied, 82.384 s, 65.2 MB/s | | |__ $ mkswap /mnt/resource/swapfile_1 .................................................................... Mark this file as a swap file. | | mkswap: /swapfile: warning: don't erase bootbits sectors on whole disk. Use -f to force. | Setting up swapspace version 1, size = 5242876 KiB | no label, UUID=1b473708-e071-4a69-ab31-1334dd987c4d | | |__ $ swapon /mnt/resource/swapfile_1 .................................................................... Mount it on the file system, and make it alive immediately. | | |__ $ swapon -s .......................................................................................... Verify if swap file got created. | | Filename Type Size Used Priority | /mnt/resource/swapfile file 2097148 0 -1 | /mnt/resource/swapfile_1 file 5242876 0 -2 | | |__ $ vi /etc/fstab ...................................................................................... Update FS table with newly added swap file. /mnt/resource/swapfile_1 swap swap defaults 0 0 <4> Creating swap partition: | |__ $ sudo su - | |__ $ fdisk -l | |__ $ mkswap /dev/hdb1 | |__ $ swapon /dev/hdb1 | |__ $ swapon -s | |__ $ vi /etc/fstab <5> Reference: | |__ Author: Gary Sims, Subject: All about Linux swap space, Source - https://www.linux.com/news/all-about-linux-swap-space
Your Comments