Guaranteed RAM vs Burstable Memory for VPS Web Hosting Servers
Many VPS web hosting providers offer both guaranteed and burstable RAM/memory limits. As of September 2008 WiredTree no longer does, but they USED TO offer » "256-MB Guaranteed RAM & 1024-MB Burstable RAM" for their lower VPS plan, and "512-MB Guaranteed RAM & 2048-MB Burstable RAM" for their top VPS plan.
I don't know if this means WiredTree now configures their servers to eliminate bursting, but they no longer market it on their web site.
Along these lines, they also eliminated their 256-MB plan, citing "stability issues" with sites/accounts using such little memory. Their minimum memory configuration now comes with » 512-MB.
The term Burstable refers to a short-term spike in memory usage, typically not seen in daily monitoring graphs. Any RAM your account does not use is available for use as 'burstable' memory by other accounts/sites hosted on your physical server ("hardware node").
If you web host offers burstable memory, that means they are using either Virtuozzo, or its open source cousin » OpenVZ, because hypervisors (like Xen and VMware) do not support bursting.
With a hypervisor, you get what you pay for .. (and not a single byte more). If your account happens to run out of memory, too bad for you. Your server will start killing processes.
Personally, I prefer the hard limits associated with non-bursting configurations. I mean, I actually like the idea of my site/account being able to burst when needed (using other-people's memory). But I don't like the idea of anybody else using *my* memory. [Piggy-wiggy me. =) ]
Every account on a hypervisor comes with fixed memory limit, which happens to be a major attraction .. knowing that no other account can steal (use) your memory, and that your web host can't allocate part of your memory to another account.
Say, for example, you're paying for a plan that includes 512-MB RAM. And let's say your account begins by using close to that amount. Then one day your account suddenly starts using much less memory (.. say 256-MB). Is this cuz your VPS host re-configured your account to give 256-MB of your memory to another account? (to increase their profit$)?
The downside to hypervisors is that they don't use memory as efficiently as Virtuozzo. Yet some feel this (efficient use of memory) leads to increased disk I/O problems (bottlenecks) with Virtuozzo.
Before concluding this section, let me say that your web hosting provider (not you) has control over the settings which allocated memory / RAM to your account. Far as I know, there is no control panel (or other status page) where you can go to see exactly HOW MUCH memory they've assigned to your account.
I did have one experience with WiredTree where the server was responding sluggishly. Response to my tickets to tech spport revealed processes/services were being killed (shut down) cuz I'd run out of memory. When I checked my memory usage in their proprietary 'Client Portal,' it was no where near the max (384-MB at the time).
I asked how I could run out of memory when my account was using no where near my (supposedly) allocated amount (384-MB). Never did receive an answer to that question. Since they'd been so supportive the rest of the time, I didn't press the issue. But obviously somebody had reset my memory allocation limit to much less than it should have been assigned.
In the past, WiredTree told me they set RAM-usage to LOWER than max-allowed .. to avoid killing processes on a short-term spike. "A safety margin," if you will. Tho they never told me HOW much lower (than 384-MB).
I admit I'm sensitive to resource usage after dealing with Lunarpages, while trying to lower our usage .. after they booted us off their production server. (And banished us to their 'Stabilization server.')
The bottom line is » your web host controls this setting, and you have no way to verify they're telling you the truth. So, if you don't feel like you can TRUST your we host, that's not a good place to be. Because you have little more than trust to go on.
On the next page, we'll look at what might be the biggest problem in virtualization today (related to performance & responsiveness) » disk input/output bottlenecks.