Addressing Matt's [comments] Against Xen

I would like properly address Matt Ayer's (WiseOne) continual claims that user created swap in xen can lead to disastrous situations. This doesn't make any sense from a Kernel point of view. Linux kernel treats Swap not as a additional memory, but as auxiliary storage, and will use it only sparingly. So a larger than ordinary swap has effectively zero impact on performance.

Quote:

I just don't have the time. Try this quick though. Create as many 64MB RAM Xen VPS's as you can, make a 1GB swap file inside each VPS, run a memory hogger, and then finally watch your system die due to horrific disk I/O. Not fun, I'll tell you that.

Conclusion: User controlled swap in a shared environment is a very, very, very bad thing.


Now let us assume there is a hostile user who is egregiously bent on creating excess disk I/O for the host and has purposefully hired vps to enact his revenge on the provider. Further assume that, for this nefarious purpose he has taken a 64MB xen and is running a full 1GB real memory allocated workload (To get create such a load itself would be difficult unless he writes his own programs to do this). Now note that, for the offending vps, the impact of the disk I/O is at the memory level, while the impact on the host is at the disk level. So very trivially, the vps would be so crippled to make it pretty much useless much before this starts having significant effect on the main server. And if engendering excess disk I/O is the sole purpose of user, then all he need to do is write a program to consecutively load and free all the files in the entire harddisk. This will create worse disk I/O than the convoluted method of using large swap. And the latter can be done on both virtuozzo and Xen--though I think in recent versions both have methods to throttle disk I/O.

The problem that UnixShell ran into with Xen was owing to their use of Snapshot, which is something that's prohibited, unless of course, you are running on a desktop with a single virtual machine. LVM snapshot will exactly double the disk I/O. The real problem that the usage is insidious and will not affect the actual vps that's leading to the excess load, but will affect the entire server as a whole.

Xen does have some drawbacks, but it can have all the important features which most providers here think are unique to virtuozzo.