KVM Virtualization Performance With Linux 2.6.31

[mtippett]

What about a comparison of different virtual machines, let's say:

qemu-kvm vs. VirtualBox (with guest additions) vs. VMWare client (with guest additions).

By the way. Was the virtual harddisk a file or a block device?

That would be lots of fun with each camp saying you need to tune each host and each guest further to make it a fair comparison . Hell, throw in Xen just for good measure .

Matt

[mtippett]

That single case was brought up as a peculiarity, not as a recommendation to use SQLite under VM. It kinda jumps up when you've immensively better results in VM than native, that's what I meant with that first bit. It was a single case where VM seemed faster so I picked it up. Naturally native being tons faster than VM is uninteresting (except for CPU-bound tasks) since it's expected. What I continued with was that I intuitively assumed that reading an SQLite database file inside an image file on the hard disk would be slower than just reading a file on the hard disk. (on latter thoughts I'm not even sure if this matters, might not even significantly increase fragmentation)
Also read my message again: My conclusion was that virtual I/O needs to develop, not that real I/O needs to take example of virtual I/O. Even though the basis for my conclusion was wrong, the conclusion was apparently right.

You've captured my personal interest in benchmarking with this post.

1) Benchmarking for numerical comparisons is somewhat borderline and interesting
2) Outliers bear particular interest and investigation
3) Investigation identifies areas for improvement/correction/isolation

PTS has a growing list of lots of these cases, where there is a "disbelief -> anger -> investigation -> acceptance -> change" cycle.

Matt

[mtippett]

For those people who have been on this thread, I dug around with the KVM and Ubuntu qemu-kvm maintainers.

It looks like "write-back" caching is turned on by default vs the recommended "write-through".

This increases performance and usablity, but it appears that it ultimately ignores requests for synchronous fileIO. Although this is the default configuration of Ubuntu currently, it effectively renders Ubuntu as not suitable for high-reliability workloads.

I have raised a defect in launchpad

https://bugs.launchpad.net/ubuntu/+s...vm/+bug/437473

So the test was useful, and since it was clearly different than the other results, it has borne value by investigating.

Michael's assertions within the article are still correct. If you want a default SQLite install to absolutely fly, run it under KVM under Ubuntu. But be aware of the risk to your data - I personally don't expect Michael to invest in the way that I did to understand and present data for each and every unusual result.

Regards,

Matthew

[VinzC]

Apart from raising a defect in Ubuntu, I see no interest in this benchmark. Who might see some interest in virtualization but companies? These companies would certainly not waste their forces in a system that is a) not fine tuned for virtualization at its best and b) based on a defect making the system unreliable. Maybe I'm too negative but that's how I see it.

[mtippett]

Apart from raising a defect in Ubuntu, I see no interest in this benchmark. Who might see some interest in virtualization but companies? These companies would certainly not waste their forces in a system that is a) not fine tuned for virtualization at its best and b) based on a defect making the system unreliable. Maybe I'm too negative but that's how I see it.

I personally use virtualization to run Windows within a window under Linux. I switch between Vista and XP by just starting an image. With virtualbox, you can also access USB which gives you driver access to USB devices.

There are other reasons such as "near-disposable images" that you can copy rather than re-install, there is also the experimentation options by installing into a VM too.

These is definitely a user-oriented thing rather than corporate.

The trigger for the caching policy change was users complaining about performance. So I can't speak for Ubuntu, but there is a reasonably strong pull for them to invest.

Regards,

Matthew

[cowmix]

For those people who have been on this thread, I dug around with the KVM and Ubuntu qemu-kvm maintainers.

It looks like "write-back" caching is turned on by default vs the recommended "write-through".

This increases performance and usablity, but it appears that it ultimately ignores requests for synchronous fileIO. Although this is the default configuration of Ubuntu currently, it effectively renders Ubuntu as not suitable for high-reliability workloads.

I have raised a defect in launchpad

https://bugs.launchpad.net/ubuntu/+s...vm/+bug/437473

The bug now says the problem should be fixed for the release.

So the test was useful, and since it was clearly different than the other results, it has borne value by investigating.

Michael's assertions within the article are still correct. If you want a default SQLite install to absolutely fly, run it under KVM under Ubuntu. But be aware of the risk to your data - I personally don't expect Michael to invest in the way that I did to understand and present data for each and every unusual result.

Regards,

Matthew

[mtippett]

The bug now says the problem should be fixed for the release.

Well, it is a lot more complex than that.

After initial disbelief, the KVM team eventually accepted that it was a current version, reproduced it themselves.

http://thread.gmane.org/gmane.comp.e...vm.devel/41353


The "closure" of the bug was really just a knee jerk reaction by Anthony because he didn't want to look deeper into the issue because he did not believe it was valid.

In the end, cooler heads prevailed, they accepted there was an issue, created and applied the patch

http://thread.gmane.org/gmane.comp.e...vm.devel/41592

I didn't bother going back to re-open and fight the launchpad bug there.

FWIW, the issue is that fdatasync wasn't actually doing anything on the some filesystems, so this now makes sure it does something. As always, if it looks free and too good to be true, it usually is.

Lots of other subtle lessons were learnt, but most of those are about human nature and communication.

Regards,

Matthew

[cowmix]

Well, it is a lot more complex than that.

After initial disbelief, the KVM team eventually accepted that it was a current version, reproduced it themselves.

http://thread.gmane.org/gmane.comp.e...vm.devel/41353


The "closure" of the bug was really just a knee jerk reaction by Anthony because he didn't want to look deeper into the issue because he did not believe it was valid.

In the end, cooler heads prevailed, they accepted there was an issue, created and applied the patch

http://thread.gmane.org/gmane.comp.e...vm.devel/41592

I didn't bother going back to re-open and fight the launchpad bug there.

FWIW, the issue is that fdatasync wasn't actually doing anything on the some filesystems, so this now makes sure it does something. As always, if it looks free and too good to be true, it usually is.

Lots of other subtle lessons were learnt, but most of those are about human nature and communication.

Regards,

Matthew

So will the fix make it into the final release of 9.10? I

[mtippett]

So will the fix make it into the final release of 9.10? I

Not sure, I would say it is probably too late (2 weeks before release is a bit late).

It's unclear if Dustin believed it was a concern or not.