View source for Swap

Can you run Linux without swap space? Yes. Should you. I would say "no". Why? I'd rather not argue the topic. Do the research yourself. Or, learn the hard way. Here is some cookbook material...

I'll rewrite and expand this page, as it's important, and I am now learning that there is a deficit of understanding about this in the field.

Some applications will require swap to run. Other than that, you don't need swap until you need it. If you don't have it when you need it, "bad things will happen" (TM). It will be too late then, and you'll be having a production emergency of some sort. Better to have some swap. One solution that is often favored because performance is seen as an issue is to use a ramdisk or something of the sort. However, if you've got enough memory and are in the "I don't need swap" camp, then what do you care? Set up your emergency stash of swap on disk. Just sayin'.


=== What is swap space, and why use it? ===

Swap space is disk space used by Linux (and most other operating systems) to store objects from memory when real physical memory starts becoming crowded or is exhausted. Because disk is generally slower than memory, Linux stores in swap the least recently used objects first, and keeps as much as it can in memory. It's generally recommended to have a swap space equal in size to your memory, if possible. See more details about swap space [https://help.ubuntu.com/community/SwapFaq here].

=== Where should I set this up? ===

The Ubuntu 12.04 LTS AMI, which I also started with, is configured with no swap space initially, as the size and number of your storage devices can vary. But most come with a large free ephemeral storage device automatically. Since S3 storage is limited by cost, the ephemeral drive is a good place for a swap file. Mine has the same device name mentioned in the question, /dev/xvdb, but you can configure this during instance launch.

=== How can I set up a swap space on my ephemeral storage drive? ===

Swap space can be set up in a single file or on a partition of a device. A file can be represented by non-adjacent blocks on a disk, whereas a partition is a predefined set of adjacent blocks. Since disk read-write is faster you don't have to keep moving the read head long distances, we get better performance using a partition. (This is why the question above discounts using a file as a temporary solution.)

Linux comes with many programs to manage disk partitions, including fdisk, sfdisk, parted, etc. We will be using sfdisk, because it can accept all the necessary arguments from a shell script. This is important, because ephemeral storage is lost every time we "stop" our instance. Thus, we set up a script in a file that automatically runs each time the instance is started, /etc/rc.local.

<pre>
# Unmount the drive in-case it is already mounted. Umount throws an error if
# it wasn't mounted, so we add || : to continue the script in that case
umount /dev/xvdb || :

# Each line below is a partition (4 maximum master partitions for this partition 
# type). Can can generally use the default arguments, supplying only the amount
# of space we want in blocks (512 came out to ~4gb for me), and the partition
# type (82 for swap, 83 for general linux is default). This will create:
# dev/xvdb1 with 1024 blocks
# dev/xvdb2 with the remainder of the disk
sfdisk /dev/xvdb << EOF
,512,82
;
;
;
EOF

# Now we format the swap partition:
mkswap /dev/xvdb1

# And the remainder. You can choose amoung the various filesystem types, but
# make sure you have the necessary formatter installed. To check, ls /sbin/mk*
mkfs.ext4 /dev/xvdb2

Finally, I preferred to include my mounting information in /etc/fstab, which is a system file on Ubuntu that prescribes how to handle various available devices automatically. It is also run at start up.

/dev/xvdb1      swap    swap    sw,nobootwait      0 0
/dev/xvdb2      /mnt2   ext4    defaults,nobootwait        0 0
</pre>

The nobootwait option is to ensure Ubuntu doesn't hang on boot with "The disk drive for /dev/xvdb(1/2) is not ready yet or not present. Continue to wait, or Press S to skip mounting or M for manual recovery."

Make sure you create a folder at /mnt2 or wherever you plan on mounting this with mkdir.

=== How do I debug the problems I'm experiencing with this? ===

You can run the rc.local script with sudo /etc/rc.local to watch it run an look for problems. sudo fdisk -l should show your new partitions after it runs. If that looks ok, try mounting your devices with sudo mount /dev/xvdb1. This will use the configuration you've saved in fstab. If that fails, try playing around with your mounting parameters and adjust fstab accordingly.