Thanks for the clarification on this. I've realised my mistake now - I need to
configure the storage array to report the LUNs as larger than they physically
are (to account for the expected de-dup ratio). I was expecting oVirt to
magaically know about this, which when thinking it through is not really
technically possible.
---- On Mon, 24 Feb 2020 13:34:49 +0000 Nir Soffer <[email protected]> wrote
----
On Mon, Feb 24, 2020 at 3:03 PM Gorka Eguileor <mailto:[email protected]>
wrote:
>
> On 22/02, Nir Soffer wrote:
> > On Sat, Feb 22, 2020, 13:02 Alan G <mailto:[email protected]> wrote:
> > >
> > > I'm not really concerned about the reporting aspect, I can look in the
> > > storage vendor UI to see that. My concern is: will oVirt stop
> > > provisioning storage in the domain because it *thinks* the domain is
> > > full. De-dup is currently running at about 2.5:1 so I'm concerned that
> > > oVirt will think the domain is full way before it actually is.
> > >
> > > Not clear if this is handled natively in oVirt or by the underlying lvs?
> >
> > Because oVirt does not know about deduplication or actual allocation
> > on the storage side,
> > it will let you allocate up the size of the LUNs that you added to the
> > storage domain, minus
> > the size oVirt uses for its own metadata.
> >
> > oVirt uses about 5G for its own metadata on the first LUN in a storage
> > domain. The rest of
> > the space can be used by user disks. Disks are LVM logical volumes
> > created in the VG created
> > from the LUN.
> >
> > If you create a storage domain with 4T LUN, you will be able to
> > allocate about 4091G on this
> > storage domain. If you use preallocated disks, oVirt will stop when
> > you allocated all the space
> > in the VG. Actually it will stop earlier based on the minimal amount
> > of free space configured for
> > the storage domain when creating the storage domain.
> >
> > If you use thin disks, oVirt will allocate only 1G per disk (by
> > default), so you can allocate
> > more storage than you actually have, but when VMs will write to the
> > disk, oVirt will extend
> > the disks. Once you use all the available space in this VG, you will
> > not be able to allocate
> > more without extending the storage domain with new LUN, or resizing
> > the LUN on storage.
> >
> > If you use Managed Block Storage (cinderlib) every disk is a LUN with
> > the exact size you
> > ask when you create the disk. The actual allocation of this LUN
> > depends on your storage.
> >
> > Nir
> >
>
> Hi,
>
> I don't know anything about the oVirt's implementation, so I'm just
> going to provide some information from cinderlib's point of view.
>
> Cinderlib was developed as a dumb library to abstract access to storage
> backends, so all the "smart" functionality is pushed to the user of the
> library, in this case oVirt.
>
> In practice this means that cinderlib will NOT limit the number of LUNs
> or over-provisioning done in the backend.
>
> Cinderlib doesn't care if we are over-provisioning because we have dedup
> and decompression or because we are using thin volumes where we don't
> consume all the allocated space, it doesn't even care if we cannot do
> over-provisioning because we are using thick volumes. If it gets a
> request to create a volume, it will try to do so.
>
> From oVirt's perspective this is dangerous if not controlled, because we
> could end up consuming all free space in the backend and then running
> VMs will crash (I think) when they could no longer write to disks.
>
> oVirt can query the stats of the backend [1] to see how much free space
> is available (free_capacity_gb) at any given time in order to provide
> over-provisioning limits to its users. I don't know if oVirt is already
> doing that or something similar.
>
> If is important to know that stats gathering is an expensive operation
> for most drivers, and that's why we can request cached stats (cache is
> lost as the process exits) to help users not overuse it. It probably
> shouldn't be gathered more than once a minute.
>
> I hope this helps. I'll be happy to answer any cinderlib questions. :-)
Thanks Gorka, good to know we already have API to get backend
allocation info. Hopefully we will use this in future version.
Nir
>
> Cheers,
> Gorka.
>
> [1]: https://docs.openstack.org/cinderlib/latest/topics/backends.html#stats
>
> > > ---- On Fri, 21 Feb 2020 21:35:06 +0000 Nir Soffer
> > > <mailto:[email protected]> wrote ----
> > >
> > >
> > >
> > > On Fri, Feb 21, 2020, 17:14 Alan G <mailto:[email protected]> wrote:
> > >
> > > Hi,
> > >
> > > I have an oVirt cluster with a storage domain hosted on a FC storage
> > > array that utilises block de-duplication technology. oVirt reports the
> > > capacity of the domain as though the de-duplication factor was 1:1, which
> > > of course is not the case. So what I would like to understand is the
> > > likely behavior of oVirt when the used space approaches the reported
> > > capacity. Particularly around the critical action space blocker.
> > >
> > >
> > > oVirt does not know about the underlying block storage thin provisioning
> > > implemention so it cannot help with this.
> > >
> > > You will have to use the underlying storage separately to learn about the
> > > actual allocation.
> > >
> > > This is unlikely to change for legacy storage, but for Managed Block
> > > Storage (conderlib) we may have a way to access such info.
> > >
> > > Gorka, do we have any support in cinderlib for getting info about storage
> > > alllocation and deduplication?
> > >
> > > Nir
> > > _______________________________________________
> > > Users mailing list -- mailto:[email protected]
> > > To unsubscribe send an email to mailto:[email protected]
> > > Privacy Statement: https://www.ovirt.org/site/privacy-policy/
> > > oVirt Code of Conduct:
> > > https://www.ovirt.org/community/about/community-guidelines/
> > > List Archives:
> > > https://lists.ovirt.org/archives/list/[email protected]/message/BKCQYELRW5XP5BVDPSJJ76YZD3N37LVF/
> > >
> > >
> > >
> > >
> >
>
_______________________________________________
Users mailing list -- mailto:[email protected]
To unsubscribe send an email to mailto:[email protected]
Privacy Statement: https://www.ovirt.org/site/privacy-policy/
oVirt Code of Conduct:
https://www.ovirt.org/community/about/community-guidelines/
List Archives:
https://lists.ovirt.org/archives/list/[email protected]/message/5MKAOBQVQKYKHUV42ZUZ64VQGPOM6LSV/
_______________________________________________
Users mailing list -- [email protected]
To unsubscribe send an email to [email protected]
Privacy Statement: https://www.ovirt.org/site/privacy-policy/
oVirt Code of Conduct:
https://www.ovirt.org/community/about/community-guidelines/
List Archives:
https://lists.ovirt.org/archives/list/[email protected]/message/FTSXCIM3ZMGCNPI64EHZGUZKR5EINTTV/
