What is a Swap space?
- Swap space is hard disk space that extends system RAM.
Swap space or virtual memory is hard disk
space that acts as an extension of system RAM. Of
course, due to the relative differential in data access on RAM versus hard
disk, we prefer not to use swap space if it can be avoided. Nonetheless, it is
vital to the proper functioning of a typical Linux system that some swap space be
made available.
Swap space is used when the amount of physical
memory (RAM) is full. If the system needs more memory resources and the RAM is
full, inactive pages in memory are moved to the swap space. While swap space
can help machines with a small amount of RAM, it should not be considered a
replacement for more RAM. Swap space is located on hard drives, which have
a slower access time than physical memory.
Swap space can be a dedicated swap partition (recommended),
a swap file, or a combination of swap partitions and swap files (a
swap file can be used, provided the space has been pre-allocated using a tool such
as dd).
The size of your swap should be equal to twice your
computer’s physical RAM for up to 2 GB of physical RAM. For physical RAM above
2 GB, the size of your swap should be equal to the amount of physical RAM above
2 GB. The size of your swap should never less than 32 MB.
Using this basic formula, a system with 2 GB of physical RAM would have 4 GB of swap, while one with 3 GB of physical RAM would have 5 GB of swap.
Creating Swap space As mentioned before,
a swap space can be a partition or a file with
pre-allocated space.
Setting up a Swap file
Suppose
your system RAM is 2GB. Then you want to allocate 2 GB for swap space. So
create a file anywhere for example in / as /swapfile.
Step 1: Open a shell, login as root
Step 2: Use dd command
to create the file called /swapfile. We can use dd many
ways to create the file.
[root@server ~]# dd if=/dev/zero of=/swapfile bs=1k count=2M
This will dump 2
million blocks of 1KB each into the /swapfile.
or
[root@server ~]# dd if=/dev/zero of=/swapfile bs=2k count=1M
This will dump 1
million blocks of 2KB each into the /swapfile.
or
[root@server ~]# dd if=/dev/zero of=/swapfile bs=1024 count=2097152
This will dump 2097152
blocks of 1024 each into the /swapfile.
Use any of these as you.
Step 3: Create the signature using mkswap.
[root@server ~]# mkswap /swapfile
Step 4: Add the swap space to the /etc/fstab file
to make it persistent at system reboots.
[root@server ~]# vim /etc/fstab
Add the following line
/swapfile swap swap defaults 0 0
Step 5: Activate the new swap space using swapon -a.
[root@server ~]# swapon -a
Step 6: Check the swap partition’s status using swapon -s or free -m.
[root@server ~]# swapon -s
[root@server ~]# free -m
Setting
up a Swap partition
Use fdisk or
some other partitioning program to add a partition. Set the partition id type
to 0×82 (Linux
Swap / Solaris). Create the signature on the partition using mkswap and
set a signature label to identify it consistently if disk device pathnames
change.
Step 1: Open a shell, login
as root
Step 2: Open fdisk to
create the partition and change the system id.
[root@server ~]# fdisk -cu /dev/sda
Press n to add a new partition.
Now change the new partition's system id by pressing the t option.
Now save the table and
exit. Restart for it it to work properly.
Step 3: Create the signature using mkswap.
[root@server ~]# mkswap -L SWAP2 /dev/sda2
mkswap: /dev/sda2: warning: don't erase bootbits sectors
(dos partition table detected). Use -f to force.
Setting up swap space version 1, size = 10485756 KiB
LABEL=SWAP2, UUID=986049e1-c454-484e-866c-22b38cb16e7b
Step 4: Add the swap space to the /etc/fstab file
to make it persistent at system reboots.
[root@server ~]# vim /etc/fstab
Add the following line
LABEL=SWAP2 swap swap defaults 0 0
Step 5: Activate the new swap space using swapon -a.
[root@server ~]# swapon -a
Step 6: Check the swap partition’s status using swapon -s or free -m.
[root@server ~]# swapon -s
[root@server ~]# free -m
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