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One feature I think we would like to see that could benefit from LVM is some kind of multidisk support with better fault tolerance....

For example:
Say you have a node, and there are 20 vm's on it, and thats all the disk io it could take. But say you have spare cpu/ram capacity other then the diskio being used up. It would be nice to be able to add a second disk, and be able to launch 20 more vm's, located on the other disk.

If you combined them together into one file system (linear append or raid0), you could loose all 40 vm's if something went wrong. That may be more then you want to risk. If you could keep them as separate file systems or logical volumes (maybe with contigous lv's?) Each vm could only top out a spindle, but it would be much more fault tolerant to failures on the machine. I can see some cases where that tradeoff between individual vm performance and number of vm's affected by a device failure can lean in that direction.

Thoughts?

Thanks,
Kevin
________________________________________
From: Premysl Kouril [premysl.kouril at gmail.com]
Sent: Tuesday, January 19, 2016 4:40 AM
To: OpenStack Development Mailing List (not for usage questions); mbooth at redhat.com
Subject: Re: [openstack-dev] [Nova] sponsor some LVM development

Hi Matt,

thanks for letting me know, we will definitely do reach you out if we
start some activity in this area.

To answer your question: main reason for LVM is simplicity and
performance. It seems from our benchmarks that LVM behavior when
processing many IOPs (10s of thousands) is more stable than if
filesystem is used as backend. Also a filesystem generally is heavier
and more complex technology than LVM and we wanted to stay really as
simple as possible on the IO datapath - to make everything
(maintaining, tuning, configuring) easier.

Do you see this as reasonable argumentation? Do you see some major
benefits of file-based backend over the LVM one?

Cheers,
Prema

On Tue, Jan 19, 2016 at 12:18 PM, Matthew Booth <mbooth at redhat.com> wrote:
> Hello, Premysl,
>
> I'm not working on these features, however I am working in this area of code
> implementing the libvirt storage pools spec. If anybody does start working
> on this, please reach out to coordinate as I have a bunch of related
> patches. My work should also make your features significantly easier to
> implement.
>
> Out of curiosity, can you explain why you want to use LVM specifically over
> the file-based backends?
>
> Matt

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