Disk Input/Output I/O Bottlenecks with VPS Web Hosting Servers
Everything in the world of Virtual Private Servers revolves around memory, but disk input/output is normally the biggest bottleneck (hindrance to server performance) in virtualized servers.
This is because .. well, here's what Zac said (following 3 bullet items):
• "No virtualization software at this point has a good disk I/O scheduler. Virtuozzo 4 was supposed to have an improved one, but it is pretty much worthless."
• "We use 8-disk RAID-10 arrays in order to maximize the IOPS (I/O Operations per Second) to minimize the risk that any one VPS can choke disk I/O for the other Virtual Environments."
• "Virtuozzo, Xen, and other virtualization software are good at scheduling CPU, but weak at scheduling disk I/O. I suppose that will come in time."
If you think about it, you can see why disk input/output is such a big thing in virtualized servers.
Consider, for example, the 3 current VPS packages offered by WiredTree (or any VPS web host you like):
- 512-MB RAM + 40-GB disk space
- 768-MB RAM + 60-GB disk space
- 1024-MB RAM + 80-GB disk space
If you add the amounts of RAM and disk space listed (to get an average of the 3), you get the following totals » 2304-MB RAM + 180-GB disk space. A little more math to convert MB RAM to GB » 2304-MB / 1.024 / 1000 = 2.360-GB total RAM (for the 3 plans).
This gives us a disk space-to-RAM ratio of (180/2.360) » 76. [Over 70 times more disk space than memory]
Even if most accounts use only half their allotted disk space, that's still almost 40 times more data located on disk storage than resident in memory.
Radified currently uses 6-GB of our allotted 40 gigs. So we use ~15% of our allotted disk storage. Even with this low number, we still have ~16 times more data located on disk than will fit in memory. (6/.512/1.024 or 6/.500 = 12)
Now Apache will do a good job at caching the most frequently requested files, but you can see how the disk storage system becomes critical the minute we start talking about server responsiveness, because disk storage is orders-of-magnitude slower than RAM. (Memory is ~ 1,000X faster than disk storage.)
On the next page, we'll delve a little deeper into the world of Virtual Private Servers by discussing the differences between Managed and unManaged VPS servers. These options represent the first decision you'll need to make before signing up for your VPS account.
This is where you'll learn you don't need to be a Linux admin guru in order to run your own VPS server (as I previously thought).