Adding Swap Space in Linux Without a Reboot

So, let's say you've got a server running out of memory. Not just RAM, but swap too. Now, generally, there are a few well known ways to solve this issue.

• Close/Kill processes you don't need
• Reboot
• Add another swap partition
• Buy more RAM
• Buy more Hardware

Now, In our scenario, the first option isn't helping, the second one is just the nuclear option to the first. But we've got one huge process and it's not all active memory... it's just consuming a lot of RAM and Swap and we want it to succeed. Buying more RAM is the best idea, but this server won't take anymore, or we're not sure we'll have this workload often, so we can't justify wasting money on more hardware. We've gotta get creative before it fills up and gets OOM killed. Adding another swap partition is a great idea, but we're out of available disk partitions or drives to throw at it. However, we do have some free space on an existing partition, we can leverage that.

$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/md2               47G   11G   35G  23% /
/dev/hda1              99M   20M   74M  22% /boot

Alright, looking at a top or vmstat, we know we've got 4GB of RAM in here, and another 2GB of swap. Knowing the size of the process, we figure doubling that swap will give us plenty of overhead at the moment. Let's do this!

$ dd if=/dev/zero of=/newswap bs=32k count=64k

65536+0 records in
65536+0 records out
2147483648 bytes (2.1 GB) copied, 18.9618 seconds, 113 MB/s

$ ls -al /newswap
-rw-r--r-- 1 root root 2147483648 Nov 19 23:02 /newswap
$ mkswap /newswap
Setting up swapspace version 1, size = 2147479 kB
$ swapon /newswap

And that's it. A quick check should find that we now have another 2GB of swap space and a system that can breathe a little more.

Note: The size of the swap space is determined by the size of the file. 'bs' is the block size, and 'count' is the number of blocks. I generally stick to 32k or 64k block sizes and then adjust the count from there. 64k & 64k is 4GB, 64k and 128k is 8GB, etc.

Now, this won't stick after a reboot as is. If you'd like it to, I recommend changing the process a bit. It's the same until you've finished the mkswap command, after that instead of running swapon, open up the /etc/fstab in your favorite editor (vi /etc/fstab) and then add another swap line after the disk the file is on is listed like so:

/newswap         swap                    swap    defaults        0 0

Then you can run 'swapon -a' and it will mount ALL swap partitions.

Note: Swap automatically stripes across multiple swap partitions of the same priority. It might be useful to make swap partitions on multiple drives to allow for faster RAID-0 type speeds across drives!

Hope this helped someone out. I had to use it the other day and was able to save a long running process that was eating up RAM like candy. It finished a few hours after I put this fix in place. Since I don't run that process often, I simply removed the line from the /etc/fstab and the next time it rebooted, it was back to it's normal swap sizes. I then deleted the file and it was like nothing ever happened!