Tag: Virtual memory

And I’m back, paging myself in, so we wrap up this topic.

So with the process pages all in in their frames, let’s add some more swap. File format this time.

1- Creating a Swap file

So Linux being Linux, you can be anything, as long as you’re a file!

So let’s get started and create a file using dd. After that, the process is very similar to what we’ve already seen. Have a look below:

The only difference is the use of the -f parameter to state that swap will be in a file and not in a partition.

In this example I created a 128MB file, but we can have multiple of these or even different sizes like the example below.

2- Making swap file available on boot

After the swap files created, they can be added to fstab to be available on boot, just like a partition, and all the same rules apply.

And there you have it. Swap files instead of swap partitions. These can be a quick(er) way of adding some swap space and have the same functionality.

Se let’s swap out and make space for a new process!

 

Note: As seen in my screenshots, the swap files should be made available only to the root user. I didn’t, since this was done in a lab environment, but don’t forget to do chmod 0600 on each file.

As promised, during the previous post, let’s get nerdy and show how to make changes to your swap settings.

Let’s start from the beginning.

1- How much swap do I have and how much am I using?

First one is really simple and I bet most of you know this already.

Use the free command to identify your memory usage and in there you will have the swap.

Ex. free -m will show you the used and free memory in Megabytes.

If you don’t have the free command, you can use /proc/meminfo to gather that information.

Ex. cat /proc/meminfo | grep Swap*

 

2- Am I using a Swap disk or Swap files?

Couldn’t be easier.

Ex. cat /proc/swaps

Does it need an explanation?

3- I need a bigger swap space!

Great, let’s increase it.

Is it a virtual machine? Even better.

Let’s start by turnoff the swap disk.

In the case above would be just running swapoff /dev/sdc1.

Next, increase the space in your virtual disk, delete the old partition and make a new one.

After just format the new partition as swap. Ex. mkswap /dev/sdc1 and swapon /dev/sdc1

Don’t forget to update fstab.

4- How about having multiple swap disks?

Well, almost the same as before, but instead of resize, just add a new disk and create new swap partition.

After that the fun starts.

Create the swap file system like mkswap /dev/sdf1 and swapon /dev/sdf1.

Now you have 2 swap areas. When the first one is full, the second will be used.

Don’t forget to add to fstab for mounting on boot.

5- How to make multiple swap partitions be used simultaneously?

Swap partitions can be used simultaneously, acting like a “RAID” group. This will improve performance significantly, especially when using separate disks.

For that, the disks should be mounted using the same priority, as seen below:

In this case I defined the priority as 3, but it could be any value. If you have more, keep in mind that the higher the number, the higher the priority up to 32767.

So, after beating up swap files in this post. I’ll let it sink in, before I do another post on Swap files.

For now i’ll page out myself!

Yup. This is a tough one.
When you start getting questions about server virtual memory, you know it’s not good.

But let’s start by the basics. Swapping vs. Paging.

Swapping

Swapping is a technique where the whole memory allocated to a process gets moved from main memory (RAM) to a secondary memory type (usually hard disk).

Paging

Paging is a different memory management technique where physical memory is divided into frames and logical memory of each process is divided into pages. Pages and Frames are usually 4KB, but can be otherwise defined, although always the same size. Pages then are moved in and out of main memory depending on current needs and frame availability, rather than moving the whole address space of a process.

With the fundamental concepts out of the way, let’s get to the more juicy stuff.

Swap partition

Linux is a modern operating system and used paging for it’s memory management. This confuses people since Linux still uses the Swap partition and you’ve probably been told that you need to get it just right. No second changes. Bullsh… Well get to that soon.

I just said Linux is a MODERN operating system and there are many ways around it.
Regardless, as a rule of thumb, you should always assign double the physical memory to a swap partition. This will allow processes to use the extra “memory” if needed. You should also evaluate how your application will use memory to understand if it is worth while having a bigger or smaller swap size.
In a virtualization world, like today’s, I actually prefer to create a separate disk altogether. This will allow me to place that disk in a fast SSD LUN if I need the extra performance for the server virtual memory. That said, this is, and should be, a corner case.

Especially in virtual environments you can change the size of this partition and even add multiple partitions. This is also true for physical environments, although more laborious. (Who wants to replace physical disks these days!)

Swap files

Now, how about those swap files?
Swap files are, just like swap partitions, with the advantage of not having to create a separate partition. In fact, you can not have a swap partition altogether. Their functionality is the same as a swap partition, but they will be files in your file system. Like a swap partition, you can keep them in a separate disk for the same purpose.

And by the way, could you possibly use swap files and a swap partition simultaneously?
Of course you can, it’s Linux, you can do whatever you want … or close enough!
You could possibly use swap files to replace your swap partition entirely while you re-dimension it.
Did I mention you can have multiple swap partitions?

Tips

When creating a new Linux machine, although not necessary, it’s always preferable to reserve some space for Swap/Virtual memory, Regardless of being in the form of a partition or swap file.
Reserve at least the same amount as physical memory.
If you are running software that allocates large chunks of memory, swap is a must
If your applications are using swap and you can’t add physical memory, try adding more partitions and setting them with the same priority, this will make swap act “like a RAID 0”, as long as they are running on similar performance disks.

This is a long post, but one I think necessary.
I will be adding another blog post with the howto’s to use all techniques mentioned here.

Let’s get swapping!