On Wed, 24 Jun 2015 10:40:59 -0700 Gordon Messmer gordon.messmer@gmail.com wrote:
On 06/23/2015 08:10 PM, Marko Vojinovic wrote:
For concreteness, let's say I have a guest machine, with a dedicated physical partition for it, on a single drive. Or, I have the same thing, only the dedicated partition is inside LVM. Why is there a performance difference, and how dramatic is it?
Well, I said that there's a big performance hit to file-backed guests, not partition backed guests. You should see exactly the same disk performance on partition backed guests as LV backed guests.
Oh, I see, I missed the detail about the guest being file-backed when I read your previous reply. Of course, I'm fully familiar with the drawbacks of file-backed virtual drives, as opposed to physical (or LVM) partitions. I was (mistakenly) under the impression that you were talking about the performance difference between a bare partition and a LVM partition that the guest lives on.
However, partitions have other penalties relative to LVM.
Ok, so basically what you're saying is that in the usecase when one is spinning VM's on a daily basis, LVM is more flexible than dedicating hardware partitions for each new VM. I can understand that. Although, I could guess that if one is spinning VM's on a daily basis, their performance probably isn't an issue, so that a file-backed VM would do the job... It depends on what you use them for, in the end.
It's true I never came across such a scenario. In my experience so far, spinning a new VM is a rare process, which includes planning, designing, estimating resource usage, etc... And then, once the VM is put in place, it is intended to work long-term (usually until its OS reaches EOL or the hardware breaks).
But I get your point, with LVM you have additional flexibility to spin test-VM's basically every day if you need to, keeping the benefit of performance level of partition-backed virtual drives.
Ok, you have me convinced! :-) Next time I get my hands on a new harddrive, I'll put LVM on it, and see if it helps me manage VM's more efficiently. Doing this on a single drive doesn't run the risk of losing more than one drive's worth of data if it fails, so I'll play with it a little more in the context of VM's, and we'll see if it improves my workflow.
Maybe I'll have a change of heart over LVM after all. ;-)
Best, :-) Marko