Swap
Updated Mar 21, 2021 ·
Swap Partitions
Swap partitions are special partitions designed for processes and for the Linux system to use if it needs "already-ready" spaces before memory is available or in cases when memory runs out. Note that it is much slower than the RAM available in your system.
- SWAP is RAM that is emulated on disk.
- All Linux system should have some swap, depending on the server use.
- Swap can be created on any block device, including swap files.
- To activate swap space, use
swapon.
Rule of Thumb:
- Low-RAM servers typically has 2GB of swap space as minimum
- If you're hitting swap spaces constantly, then it might be time to upgrade the physical memory or create additional swap spaces on different partitions.
- You would need a sufficient space for kernel dump. Memory should be double, up to 2gb, and additiona 1xmemory for anything over 2gb
Example:
- If you have 8gb system, divide it into two: 2gb and 6gb.
- Then double the 2gb so you would have 4gb.
- Then you add the 6gb on top of the 4gb, so you would need a minimum of 10gb.
To see the available swap space, we can run either of these two commands:
## In my case, I dont have a swap space set in my partitions.
eden@tst-rhel:mnt $ df -h
Filesystem Size Used Avail Use% Mounted on
devtmpfs 7.7G 0 7.7G 0% /dev
tmpfs 7.8G 0 7.8G 0% /dev/shm
tmpfs 7.8G 17M 7.8G 1% /run
tmpfs 7.8G 0 7.8G 0% /sys/fs/cgroup
/dev/xvda2 10G 2.8G 7.3G 28% /
tmpfs 1.6G 0 1.6G 0% /run/user/1000
/dev/mapper/myvolumes-group1 196G 60M 186G 1% /mnt/disklvm
eden@tst-rhel:mnt $ sudo cat /proc/swaps
Filename Type Size Used Priority
Lab Setup
For this lab, I added three new EBS volumes into my EC2 instance so that I now have a total of 6 secondary disks.
- /dev/xvdb - added to logical volume group
- /dev/xvdc - added to logical volume group
- /dev/xvdd - added to logical volume group
- /dev/xvde - new disk
- /dev/xvdf - new disk
- /dev/xvdg - new disk
We will be using /dev/xvde:
$ sudo fdisk /dev/xvde -l
Disk /dev/xvde: 24 GiB, 25769803776 bytes, 50331648 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Creating Swap
There are two tools we can use to create swap partitions:
fdiskParted
Using fdisk
Create a new partition:
$ sudo fdisk /dev/xvde
Welcome to fdisk (util-linux 2.32.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Device does not contain a recognized partition table.
Created a new DOS disklabel with disk identifier 0x02a79247.
Command (m for help): n
Partition type
p primary (0 primary, 0 extended, 4 free)
e extended (container for logical partitions)
Select (default p): p
Partition number (1-4, default 1):
First sector (2048-50331647, default 2048):
Last sector, +sectors or +size{K,M,G,T,P} (2048-50331647, default 50331647):
Created a new partition 1 of type 'Linux' and of size 24 GiB.
Change type to Linux Swap:
# "l" to list the types. For linux swap, the hex code is 82
Command (m for help): l
0 Empty 24 NEC DOS 81 Minix / old Lin bf Solaris
1 FAT12 27 Hidden NTFS Win 82 Linux swap / So c1 DRDOS/sec (FAT-
2 XENIX root 39 Plan 9 83 Linux c4 DRDOS/sec (FAT-
3 XENIX usr 3c PartitionMagic 84 OS/2 hidden or c6 DRDOS/sec (FAT-
4 FAT16 <32M 40 Venix 80286 85 Linux extended c7 Syrinx
5 Extended 41 PPC PReP Boot 86 NTFS volume set da Non-FS data
6 FAT16 42 SFS 87 NTFS volume set db CP/M / CTOS / .
7 HPFS/NTFS/exFAT 4d QNX4.x 88 Linux plaintext de Dell Utility
8 AIX 4e QNX4.x 2nd part 8e Linux LVM df BootIt
9 AIX bootable 4f QNX4.x 3rd part 93 Amoeba e1 DOS access
a OS/2 Boot Manag 50 OnTrack DM 94 Amoeba BBT e3 DOS R/O
b W95 FAT32 51 OnTrack DM6 Aux 9f BSD/OS e4 SpeedStor
c W95 FAT32 (LBA) 52 CP/M a0 IBM Thinkpad hi ea Rufus alignment
e W95 FAT16 (LBA) 53 OnTrack DM6 Aux a5 FreeBSD eb BeOS fs
f W95 Ext'd (LBA) 54 OnTrackDM6 a6 OpenBSD ee GPT
10 OPUS 55 EZ-Drive a7 NeXTSTEP ef EFI (FAT-12/16/
11 Hidden FAT12 56 Golden Bow a8 Darwin UFS f0 Linux/PA-RISC b
12 Compaq diagnost 5c Priam Edisk a9 NetBSD f1 SpeedStor
14 Hidden FAT16 <3 61 SpeedStor ab Darwin boot f4 SpeedStor
16 Hidden FAT16 63 GNU HURD or Sys af HFS / HFS+ f2 DOS secondary
17 Hidden HPFS/NTF 64 Novell Netware b7 BSDI fs fb VMware VMFS
18 AST SmartSleep 65 Novell Netware b8 BSDI swap fc VMware VMKCORE
1b Hidden W95 FAT3 70 DiskSecure Mult bb Boot Wizard hid fd Linux raid auto
1c Hidden W95 FAT3 75 PC/IX bc Acronis FAT32 L fe LANstep
1e Hidden W95 FAT1 80 Old Minix be Solaris boot ff BBT
Command (m for help): t
Selected partition 1
Hex code (type L to list all codes): 82
Changed type of partition 'Linux' to 'Linux swap / Solaris'.
Command (m for help): v
Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.
To check, print the details:
$ sudo fdisk /dev/xvde
Welcome to fdisk (util-linux 2.32.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Command (m for help): p
Disk /dev/xvde: 24 GiB, 25769803776 bytes, 50331648 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x02a79247
Device Boot Start End Sectors Size Id Type
/dev/xvde1 2048 50331647 50329600 24G 82 Linux swap / Solaris
Next, set up swap space using mkswap:
$ sudo mkswap /dev/xvde1
Setting up swapspace version 1, size = 24 GiB (25768751104 bytes)
no label, UUID=e6bcc068-628c-4555-b06e-9cda9563cf8c
Turn on/off swap space using swapon/swapoff:
$ sudo swapon /dev/xvde1 -v
swapon: /dev/xvde1: found signature [pagesize=4096, signature=swap]
swapon: /dev/xvde1: pagesize=4096, swapsize=25768755200, devsize=25768755200
swapon /dev/xvde1
Checking the /proc/swaps directory, we can see that a file has been created:
$ cat /proc/swaps
Filename Type Size Used Priority
/dev/xvde1 partition 25164796 0 -2
If we turn off swap, it would disappear from /proc/swap.
sudo swapoff
$ cat /proc/swaps
Filename Type Size Used Priority
Note that if we reboot the system, the configuration we did wouldn't persist. We need to add an entry to fstab.
Using parted
I removed the old EBS volumes and created two additional EBS volumes, attached them to my instance, and reused the same disk names - xvdb and xvdc.
$ lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
xvda 202:0 0 50G 0 disk
├─xvda1 202:1 0 1M 0 part
└─xvda2 202:2 0 50G 0 part /
xvdb 202:16 0 10G 0 disk
xvdc 202:32 0 9G 0 disk
I then created a label and partition on it using parted:
(parted) mklabel gpt
(parted) mkpart one 1GB 2GB
(parted) PRINT
Model: Xen Virtual Block Device (xvd)
Disk /dev/xvdb: 10.7GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:
Number Start End Size File system Name Flags
1 1000MB 2000MB 999MB one
This time, we'll use mkpart in interactive mode to create the swap partition. It's crucial to specify the filesystem type for swap space as linux-swap, along with setting where the partitions starts (2GB) and where it ends (3GB).
(parted) mkpart
Partition name? []? swap
File system type? [ext2]? linux-swap
Start? 2GB
End? 3GB
(parted) print
Model: Xen Virtual Block Device (xvd)
Disk /dev/xvdb: 10.7GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:
Number Start End Size File system Name Flags
1 1000MB 2000MB 999MB one
2 2000MB 3000MB 1000MB linux-swap(v1) swap
(parted) quit
Information: You may need to update /etc/fstab.
Next step is to format the partition to use swap. We can use mkswap to do this.
[root@tst-rhel ~]# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
xvda 202:0 0 50G 0 disk
├─xvda1 202:1 0 1M 0 part
└─xvda2 202:2 0 50G 0 part /
xvdb 202:16 0 10G 0 disk
├─xvdb1 202:17 0 953M 0 part
└─xvdb2 202:18 0 954M 0 part
xvdc 202:32 0 9G 0 disk
[root@tst-rhel ~]#
[root@tst-rhel ~]# mkswap /dev/xvdb2
Setting up swapspace version 1, size = 954 MiB (1000337408 bytes)
no label, UUID=b6b96d62-904b-49d8-b186-396d8f6d3d6d
Before we enable the swap, we need to first check the free memory. This should change after running swapon.
[root@tst-rhel ~]# free -m
total used free shared buff/cache available
Mem: 15829 207 15243 1 379 15359
Swap: 0 0 0
[root@tst-rhel ~]# swapon /dev/xvdb2
[root@tst-rhel ~]# free -m
total used free shared buff/cache available
Mem: 15829 208 15242 1 379 15358
Swap: 953 0 953
Persistent Swap
Before we do anything, make sure /dev/xvdb2 is not mounted.
[root@tst-rhel ~]# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
xvda 202:0 0 50G 0 disk
├─xvda1 202:1 0 1M 0 part
└─xvda2 202:2 0 50G 0 part /
xvdb 202:16 0 10G 0 disk
├─xvdb1 202:17 0 953M 0 part
└─xvdb2 202:18 0 954M 0 part
xvdc 202:32 0 9G 0 disk
Let's add an entry to the /etc/fstab for the swap partition. Notice that it will not be mounted to any directory but instead directly to swap.
[root@tst-rhel ~]# vim /etc/fstab
UUID=d35fe619-1d06-4ace-9fe3-169baad3e421 / xfs defaults 0 0
UUID=e6bcc068-628c-4555-b06e-9cda9563cf8c swap swap defaults 0 0
/dev/xvdb2 swap swap defaults 0 0
Mount the filesystem:
sudo mount -a
Checking the block devices again:
[root@tst-rhel ~]# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
xvda 202:0 0 50G 0 disk
├─xvda1 202:1 0 1M 0 part
└─xvda2 202:2 0 50G 0 part /
xvdb 202:16 0 10G 0 disk
├─xvdb1 202:17 0 953M 0 part
└─xvdb2 202:18 0 954M 0 part [SWAP]
xvdc 202:32 0 9G 0 disk