Does your application depend on, or report, free disk space? Re: F20 Self Contained Change: OS Installer Support for LVM Thin Provisioning

[Mike Snitzer]

On Wed, Jul 31 2013 at  5:53pm -0400,
Chris Murphy <lists at colorremedies.com> wrote:

> 
> On Jul 31, 2013, at 12:38 PM, Eric Sandeen <sandeen at redhat.com> wrote:
> 
> > 
> > i.e. if you only want the efficient snapshots, a way to fully-provision
> > a "thinp" device.  I'm still not sure if this is possible…?
> 
> […]
> 
> > 
> > I guess I'm pretty nervous about offering actual thin provisioned
> > storage to "average" Fedora users.  I'm having nightmares about the "bug"
> > reports already, just based on the likelihood of most users misunderstanding
> > the  feature and it's requirements & expected behavior…
> 
> So possibly the installer should be conservative about how thin the
> provisioning is;

We (David Lehman, myself and others on our respective teams) have
already decided some months ago that any thin LVs that anaconda
establishes will _not_ oversubscribe the thin-pool.

And in fact a reserve of free space will be kept in the thin-pool as
well as the parent VG.

> otherwise I'm imagining inadequately provisioned thinp LV, while also
> using the rollback feature [1].

Can you elaborate?  Rollback with LVM thin provisioning doesn't require
any additional space in the pool.  It is a simple matter of swapping the
internal device_ids that the thin-pool uses as an index to access the
corresponding thin volumes.  This is done when activating the thin
volumes.

LVM2's support thinp snapshot merge (aka rollback) is still pending, but
RFC patches have been published via this BZ:
https://bugzilla.redhat.com/show_bug.cgi?id=957881
 
> [1] https://fedoraproject.org/wiki/Changes/Rollback

The Rollback project authors have been having periodic concalls with
David Lehman, myself and others.  So we are relatively coordinated ;)

Mike