[openstack-dev] [Nova] sponsor some LVM development

James Bottomley James.Bottomley at HansenPartnership.com
Tue Jan 19 15:51:29 UTC 2016

On Tue, 2016-01-19 at 13:40 +0100, Premysl Kouril wrote:
> Hi Matt,
> thanks for letting me know, we will definitely do reach you out if we
> start some activity in this area.

You still haven't answered Anita's question: when you say "sponsor" do
you mean provide resources to existing developers to work on your
feature or provide new developers.

> To answer your question: main reason for LVM is simplicity and
> performance.

Heh, this is history repeating itself from over a decade ago when
Oracle would have confidently told you that Linux had to have raw
devices because that's the only way a database will perform.  Fast
forward to today and all oracle databases use file backends.

Simplicity is also in the eye of the beholder.  LVM has a very simple
naming structure whereas filesystems have complex hierarchical ones. 
 Once you start trying to scale to millions of instances, you'll find
there's quite a management penalty for the LVM simplicity.

>  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.

It sounds like you haven't enabled directio here ... that was the
solution to the oracle issue.

>  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.

And this was precisely the Oracle argument.  The reason it foundered is
that most FS complexity goes to manage the data structures ... the I/O
path can still be made short and fast, as DirectIO demonstrates.  Then
the management penalty you pay (having to manage all the data
structures that the filesystem would have managed for you) starts to
outweigh any minor performance advantages.


> 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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