Creating a Swap File for Your VPS

Introduction

RAM is expensive when compared to physical hard disk (storage) in today’s market. Combine this knowledge with Virtual Private Server (VPS) solutions offered by vendors such as Amazon EC2 who will literately give us our own Linux box in The Cloud. They’ll also grant us full root level privileges to do whatever we want in these new sandboxes of ours. These vendors usually provide their product to us with a ton of disk space and very little RAM. That said, the more cash we’re willing to shell out of our wallet, the more ram and disk space we’ll receive in return (just like everything else in our consumer driven world).

If you chose a VPS as your preliminary (or final) hosting solution, then you (like everybody else) are on a budget. There is a good chance you’re working with a very minimal amount of RAM (probably 512MB to 4GB) regardless of how much cash you forked out of your pocket. Now you could argue that this is all you need, but this is where I say: you still need swap space no matter what.

This is truly worth blogging about because if you install just about any Linux distribution, it will (usually) set up a swap partition for you as part of it’s default out of the box settings. But most VPS solutions out there provide you with a pre-configured distribution of Linux that is not equipped with any swap space whatsoever. I wrote this blog because popular VPS vendors such as Linode and Digital Ocean do not provide swap space with their virtual machine (or at least at the time of writing this).

You were saying…

Did you ever think that maybe they don’t provide it because you don’t need it?

No, that’s simply not the case. The fact of the matter is; they offer packages on their website such as 1GB of RAM and 20GB of storage space and that is exactly what they give you. Could you just imagine how frustrated you’d be as a customer if you paid for that deal and found out you actually only had 512MB (0.5GB) of physical RAM while the remaining 512MB was just your swap partition? That would be just false advertising in my opinion. The issue could get even more complicated then that… would that mean they’d be providing you with 20GB of storage space minus (-) 512MB because it’s reserved for this swap partition? Thankfully at the end of the day we get what we pay for. In this example we’d have purchased a Linux distribution with 1 root partition (/) that is exactly 20GB in size along with 1GB of physical RAM. We could type the command free -m and see this 1GB of physical RAM for ourselves.

Why do I need swap space?

  • You grant your system more (virtual) memory (RAM) to work with. This is especially useful in dire situations when your memory may be almost exhausted. Remember, if you run out of RAM, the services running on your server WILL start to fail on you.
  • It can save you money! Why opt for a more expensive VPS solution that comes with more RAM when your application you choose to host (A mail server for example) may only utilize all of the RAM during it’s start up. Once the application has finished starting up, it may only use 2GB of ram thereafter. Was it really worth spending $100+ more a year just to handle your applications startup? A swap disk can alleviate this for you allowing you to choose a cheaper server.
  • You will free up physical RAM while swap space is present.

    Consider this: you’re running a program that has 200 features in it, but you’re only using 10 of them. Without swap space, all 200 features may (and most probably will) be loaded into physical (expensive) memory; even the features you’re not using! But with a swap file/partition, the system will detect the sections of the code not being referenced/used (in RAM) and move it into swap space. This process frees up physical memory for other services you want to run. The program won’t care you’re doing this; as far as it’s concerned, the resources are still available to it (which they are).

    In other words: The operating system is smart enough to note the frequency of code being executed. These pieces of accessed code will always remain in memory while less frequently accessed code will be swapped to the (swap) disk until it is needed again. The process is called paging when the operating system goes back into the swap space to retrieve this data (returning it to RAM for the program that requested it). We have some control over the swapping process (to limit paging) which I talk about later (swappiness).

Swap files are slow and can cause extensive i/o:

Swapping content from RAM back to a file storage (and back) is slow; there is no question about this. I also will not argue with you that this process does generate i/o. You obviously want to minimize the swapping of the content that you’re quite sure will be used again. But you still never want to eliminate it completely as it plays a huge role in optimizing your system.

In fact, the cons of swap space only outweigh it’s benefits if you abuse it by treating it as if it were additional physical RAM.

Hence you DO NOT create virtual swap space because you are out of memory! You create a swap space to optimize system resources. You create swap space to provide a safety net for your applications during peak operation times. You create swap space to increase system performance by freeing up sections of memory that are rarely being used.

If you only paid for 0.5GB (512MB) of RAM from your VPS provider, then do not consider yourself as having 2.5GB of RAM after you create a 2GB swap file. Sure, let your applications think this, but you yourself don’t want to exceed this 0.5GB memory boundaries any more then you have to. Obviously you’re creating more (virtual) RAM so that you can venture into the higher ranges in dire situations, but thats it. Otherwise you’re not using swap space effectively.

A Final Word of Caution…

With some vendors such as Amazon EC, you’ll pay for the disk i/o you generate. So improper use of swap space would work against you (costing you). Meanwhile companies like Crissic Solutions go out of their way to micro-manage the virtual machines they host. They specifically look for people violating disk i/o unnecessarily and are quick to launch their own internal investigations to see if you’re abusing their privileges. It’s this tight environment they enforce on everyone that allows them to promise their clients a very responsive experience.

Word of Caution

Just a Final Word of Caution

If you decide you need swap space (as I did) along with your services (and you should), then do not treat your results as free RAM.

The decision you make in choosing how much extra RAM you will give yourself with a swap file will not decide if you intend to abuse your hosting privileges. However the amount of this new free memory you intend to exhaust at once could… so just be considerate! Most vendors will generally just trust that you are not doing anything malicious; in fact you acknowledged this as part of the Terms of Service when you first signed up. So with that said, please don’t abuse the information in this blog or your VPS provider WILL catch you if their system disk i/o spikes for long durations of time. Trust me when I say: “it won’t take rocket science to track the source to be your VPS if you abuse their services.”

Its also worth noting that if you already have a swap partition on your system, then it’s really not worth adding another. You probably want to consider adding or paying for more RAM if your system is still struggling.

How much swap space do I need?

This really is the magic question, there are lots of people who have tons of different considerations for this. Personally, it honestly depends on what your going to run. Some applications utilize an insane amount of ram up front, but once they’re past their start up phase, they only use a fraction of RAM. Meanwhile, other systems are just generally memory hogs.

At the end of the day, the size really isn’t important. In fact it’s probably safer to have to much then too little. I mean if you had 16GB of physical RAM in your VPS, I’d (arbitrarily) say create a 10GB swap partition. This allows you to run a TON of applications and all of them have plenty of space to dump their unused code to. It also leaves you with a bit of a buffer (of extra RAM) in critical situations. If you have 512MB to 4GB of physical RAM, then I’d say create a partition that is about 2GB in size. Disk space is cheap and 2GB really isn’t that much.

Again, the decision is really up to you… more space is better then less (but too much just becomes wasted disk space). There is also no reason for your swap space to ever be larger then your actual alloted physical RAM. You’ll want to watch and make sure that too much paging is taking place at the end of the day regardless of what size you pick. Excessive paging means you’ve exhausted your physical RAM and you’re relying on the swap space you created to fill the void. You need to consider paying for more RAM at this point or not running so many applications on the Virtual Machine your VPS provided you.

Commands

Here is how you can create your own swap space (as a swap file) on an already running system.

# Creating a directory to host our swap file
mkdir -p /var/swap

# Agree on a size... in my case I want a 2GB buffer.
# Now ideally this would be the easy to read command to run
# as the following:
#   dd if=/dev/zero of=/var/swap/swap.fs bs=2G count=512
#
# But.. no... it's not that simple, the above command would
# need 2GB of ram which you may or may not have (yet).... so we
# need to work with a smaller block size and a higher count to
# achieve the same results.
#
# The below command is more i/o intensive but less memory
# intensive which is the situation for must of us reading this today.
#
# 1024MB = 1GB
# (1024 * 1024MB * 2) = 2097152 block size
#
dd if=/dev/zero of=/var/swap/swap.fs bs=1024 count=2097152

# Format swap file (allowing it to be a recognizable swap space
# partition)
mkswap -L swapfs /var/swap/swap.fs

# You may see a warning about including -f; don't bother... you're
# done after the above command, Now you want to protect
# the new file you created
chmod 600 /var/swap/swap.fs
chmod 100 /var/swap

# Update fstab file:
echo '/var/swap/swap.fs swap swap defaults 0 0'  >> /etc/fstab

# Enable the swap partition
swapon /var/swap/swap.fs

You’re done… You can use the following command to test to see if your partition got activated or not:

# Test to see swap correctly activated itself:
free -m

Swappiness

Yes, swappiness is a word… at least it is in respect to disk swapping/paging it is. Your system is most likely set to the same level of swappiness as the rest of us have (60). Here is how you can check what yours is set to:

[root@node01 ~]# cat /proc/sys/vm/swappiness
60

Swappiness is a number from 0 to 100 where the higher values lead to more pages being swapped, and lower values lead to more applications being kept in memory, even if they are idle. Generally 60 is pretty safe to use, but if this isn’t your machine that is hosting it. You may want to tweak this value a bit. Bring it down a bit so you only swap in situations where you absolutely have to. This will minimizing the i/o impact on the server provided to you at such a great cost.

This can be done using the following command:

# Note this is temporary and will be lost on a reboot
echo 40 > /proc/sys/vm/swappiness

Or if your happy with your change, you can permanently save this change by updating your /etc/sysctl.conf file:

# first make sure an existing entry doesn't already exist
sed -i -e '/[ t]*vm.swappiness/d' /etc/sysctl.conf

# Now create it with our entry
echo "vm.swappiness=40" >> /etc/sysctl.conf

# Now future reboots will hold this setting for you

It’s not really necessary to play with the default swappiness value unless this isn’t your server you just created a swap file on. Then it becomes a nice way to minimize unnecessary i/o when using a VPS as your solution.

Credit

If you like what you see and wish to copy and paste this HOWTO, please reference back to this blog post at the very least. It’s really all I ask.

Sources

Swap file generation is really well documented everywhere and certainly isn’t a new concept. My goal was just to bring it to your attention that most VPS solutions do not provide you with any swap space at all initially and you should consider adding it right away. Here are some helpful links on swapping:

Virtual Private Server (VPS)

I made reference to a few of them in this blog; I don’t want to promote one over the other either. It’s still worth sharing the different VPS solutions out there since they are a cost-effective way to host content for your personal use or business.

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