Introduction

I have a great interest in the paging functions of the kernel. This is due to several blown hard drives which I believe where thrashed to death by page swapping and system file reads... (i.e. swap partition and root/data on same device.) So I decided to replace the typical swap set up with one that didn't rely on a mechanical device which is prone to failure. In doing so I have increased the performance of my system by 50% overall, and an astounding 200% in some cases of heavy paging activity.

How to Swap

Hardware: AMD64 +3800@2.41 Ghz 512MB RAM Mainboard with 4 external USB2.0 ports 4 identical 512MB USB2.0 storage devices.

Software: Ubuntu 7.04 Feisty Fawn

Setup procedure

Partition and format the 4 USB storage devices as linux swap. Reboot. Open gparted and check the devices are present and proper format. Edit /etc/fstab and add the following lines

/dev/sda1 none swap sw,pri=10 0 0

/dev/sdb1 none swap sw,pri=10 0 0

/dev/sdc1 none swap sw,pri=10 0 0

/dev/sdd1 none swap sw,pri=10 0 0

#Note: the device names should match what is shown by gparted.

You should also see the original swap partition with a pri value, change that too 0 if it is not already. You want to have a Hard Disk swap partition as a fall back messure. (e.g. /dev/hdc2 none swap sw,pri=0 0 0 )

save and exit. Restart machine.

Known bugs

On restart Ubuntu did not turn on the new swap devices for me, if this happen to you as well you can turn them on manually with swapon -a or place the command inside a startup script which runs before the desktop is loaded (i inserted a link into rc5.d to a script called S04swapsys).

pri=10 is not the most effective value (I still need to do some research on this), the reason is because each page is assigned a priority which is lower then the last. What I think this means is if you assign a device pri=10, the first page used has pri=9. What should happen is all swap pages are created on the USB devices first, then the HD partition is used, or the HD is used if the USB devices are busy.

While I am using AMD64, I'm not sure if the pri option accepts 64bit values yet... more research.

Info

The reason for this swap setup is to elimanate the HD bottleneck with fast USB flash memory devices. The read speeds on these devices are far faster then a HD so when a page is requested to be placed into physical memory, it happens almost instantly. Since the write speeds are much slower, multiple devices are used to multi-task the process and again, the HD swap partition is there as a fall back.

I have been testing this setup with excellent results using the following software:

-Wine-> World of Warcraft(windowed mode)

-Firefox->flash & graphic pages

-Azureus-> large file download.

-Mplayer-> playing movie

-xmms->music stream

These are all running at the same time and I am focus switching back and forth with no problems, very fast results.

WoW running by itself is where I get the 200% value. It is based on how long it takes to reload large areas and how long it takes move when entering populated cities and areas. The game runs smoother, faster and is more stable then running it on the OS it was designed for.

Also see

man swapon(2) priority option