Add Swap file
First we check out the memory usage:
[root@srv-2 /root]# free -m total used free shared buffers cached Mem: 251 242 8 22 11 32 -/+ buffers/cache: 198 52 Swap: 133 133 0 |
Make sure we have 128 MB laying around somewhere:
[root@srv-2 /root]# df Filesystem 1k-blocks Used Available Use% Mounted on /dev/hda9 132207 33429 91952 27% / /dev/hda1 15522 2537 12184 17% /boot /dev/hda6 6143236 739000 5092176 13% /opt /dev/hda7 1035660 836204 146848 85% /usr /dev/hda5 2071384 344048 1622112 17% /usr/local /dev/hda8 303344 14439 273244 5% /var |
OK, we’re going to make a swap file in /opt by using dd to create a file 128 MB in size.
[root@srv-2 /opt]# dd if=/dev/zero of=swapfile bs=1024 count=132207 132207+0 records in 132207+0 records out [root@srv-2 /opt]# ls -l total 132364 drwxr-xr-x 20 usr-3 users 4096 May 22 10:46 usr-3 drwxr-xr-x 2 root root 16384 Feb 21 07:04 lost+found -rw-r--r-- 1 root root 135379968 May 29 11:52 swapfile |
Hey, I know, let’s not make it world-readable…
[root@srv-2 /opt]# chmod 600 swapfile [root@srv-2 /opt]# ls -l total 132364 drwxr-xr-x 20 usr-3 users 4096 May 22 10:46 usr-3 drwxr-xr-x 2 root root 16384 Feb 21 07:04 lost+found -rw------- 1 root root 135379968 May 29 11:52 swapfile |
Now we set up the swap area and enable it.
[root@srv-2 /opt]# mkswap swapfile Setting up swapspace version 1, size = 135372800 bytes [root@srv-2 /opt]# swapon swapfile |
And viola! Twice as much swap as before.
[root@srv-2 /opt]# free total used free shared buffers cached Mem: 257632 254632 3000 2512 36172 15096 -/+ buffers/cache: 203364 54268 Swap: 268708 136512 132196 |
You can edit /etc/fstab to enable your swap file automatically at boot time.
By adding an entry like this:
/opt/swapfile swap swap defaults 0 0 |
Sure, swapping’s ugly, slow and will grind your hard drives to dust. But even modern systems which have been tuned for performance require a generous oodle of swap space.
How much swap space is currently used by the system?
Free command displays the swap space. free -k shows the output in KB.
# free -k
total used free shared buffers cached
Mem: 3082356 2043700 1038656 0 50976 1646268
-/+ buffers/cache: 346456 2735900
Swap: 4192956 0 4192956
Swapon command with option -s, displays the current swap space in KB.
# swapon -s Filename Type Size Used Priority /dev/sda2 partition 4192956 0 -1
Swapon -s, is same as the following.
# cat /proc/swaps Filename Type Size Used Priority /dev/sda2 partition 4192956 0 -1
How to find out which process is using swap space?
If we would like to sort out the running or queueing process as per swap usage we can do like :#topThen press capital “o” (ie “O”) followed by “p” and press enter. Now processes should be sorted by their swap usage.
We can also use bash script to pick up the process from /proc file system. So, use the following script :
—–
#!/bin/bash
# Get current swap usage for all running processes
SUM=0
OVERALL=0
for DIR in `find /proc/ -maxdepth 1 -type d | egrep “^/proc/[0-9]”` ; do
PID=`echo $DIR | cut -d / -f 3`
PROGNAME=`ps -p $PID -o comm –no-headers`
for SWAP in `grep Swap $DIR/smaps 2>/dev/null| awk ‘{ print $2 }’`
do
let SUM=$SUM+$SWAP
done
echo “PID=$PID – Swap used: $SUM – ($PROGNAME )”
let OVERALL=$OVERALL+$SUM
SUM=0
done
echo “Overall swap used: $OVERALL”
—–
Save it as getswapusage.sh and change its permission like :
#chmod 755 getswapusage.sh
Now run it like :
#./getswapusage.sh |sort -n -k 5
We can view swap usage at that particular moment by particular process.
We can also monitor total swap usage by following command :
#watch cat /proc/meminfo
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