The Limiting Factor in VPS Performance
When looking at VPS providers, keep in mind that other VPSs are running on the same machine, each competing for limited hardware resources.
However, our VPS software dynamically allocates CPU and memory using fair-share technology. Assuming you're on a PowerEdge 2650 with dual Xeons and plenty of RAM, what's the limiting factor in VPS perfomance?
Hard drives.
We've all been acutely aware of this when there's a lot of disk activity on a server – everything seems to slow to a crawl.
This means IDE disks aren't an option in our machines. We use Ultra320 SCSI drives backed by five-year warranties.
RAID - "Fast, Cheap, Reliable: Choose Two"
We install the SCSI drives in a RAID array for even faster data access. You might expect RAID-1 or RAID-5, but there's a better option. Let's first look at how well the two common RAID levels perform. (Performance characteristics are from storagereview.com.)
RAID-1 combines two disks into one, giving better performance than a single drive by itself. Compared to other RAID configurations, RAID-1 is,
- Good for sequential writes
- Fair for sequential reads
- Good for random reads and writes
RAID-5 is common because it's cheaper and gives more usable space than RAID-1. How does RAID-5 stack up?
- Fair to good for sequential writes
- Good to very good for sequential reads
- Very good to excellent for random reads
- Fair for random writes
In a RAID-1 or RAID-5 array without a hot-spare drive, your data is lost if more than one disk fails – reliability is decent.
RAID-5 is a nice compromise for some companies, but we feel that disk performance is so important to a quality VPS, we go even further.
RAID-10 - "Fast and Reliable"
RAID-10 gives you,
- Good to very good sequential write performance
- Very good to excellent sequential read performance
- Very good to excellent random read performance
- Good to very good random write performance
If you want a more specific comparison than "good to very good," RAID-10 is about 20-30% faster than RAID-5.
A RAID-10 array can potentially withstand two drive failures before losing your data, making it more reliable than RAID-1 or RAID-5.
Using RAID-5 would save us some money, but the tradeoffs aren't worth it. Based off feedback from our clients, we made the right choice.