On Wed, 24 Jun 2015 10:40:59 -0700
Gordon Messmer <gordon.messmer@xxxxxxxxx> 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
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