[ovirt-users] DISCARD support?

Nir Soffer nsoffer at redhat.com
Tue Oct 4 20:48:42 UTC 2016

On Tue, Oct 4, 2016 at 11:11 AM, Nicolas Ecarnot <nicolas at ecarnot.net>

> Hello,
> Sending this here to share knowledge.
> Here is what I learned from many BZ and mailing list posts readings. I'm
> not working at Redhat, so please correct me if I'm wrong.
> We are using thin-provisioned block storage LUNs (Equallogic), on which
> oVirt is creating numerous Logical Volumes, and we're very happy with it.
> When oVirt is removing a virtual disk, the SAN is not informed, because
> the LVM layer is not sending the "issue_discard" flag.
> /etc/lvm/lvm.conf is not the natural place to try to change this
> parameter, as VDSM is not using it.

> Efforts are presently made to include issue_discard setting support
> directly into vdsm.conf, first on a datacenter scope (4.0.x), then per
> storage domain (4.1.x) and maybe via a web GUI check-box. Part of the
> effort is to make sure every bit of a planned to be removed LV get wiped
> out. Part is to inform the block storage side about the deletion, in case
> of thin provisioned LUNs.
> https://bugzilla.redhat.com/show_bug.cgi?id=1342919
> https://bugzilla.redhat.com/show_bug.cgi?id=981626

This is already included in 4.0, added in:

However it is disabled by default. To enable discard, you need to
enable the irs:discard_enable option.

The best way to do this is to create a dropin conf:

discard_enable = true

And restart vdsm.

You need to deploy this file on all hosts.

In the next version we want to enable this automatically if the storage
domain supports discard, no configuration on the host will be needed.


> --
> Nicolas ECARNOT
> On Mon, Oct 3, 2016 at 2:24 PM, Nicolas Ecarnot <nicolas at ecarnot.net>
> wrote:
>> Yaniv,
>> As a pure random way of web surfing, I found that you posted on twitter
>> an information about DISCARD support. (https://twitter.com/YanivKaul
>> /status/773513216664174592)
>> I did not dig any further, but has it any relation with the fact that so
>> far, oVirt did not reclaim lost storage space amongst its logical volumes
>> of its storage domains?
>> A BZ exist about this, but one was told no work would be done about it
>> until 4.x.y, so now we're there, I was wondering if you knew more?
> Feel free to send such questions on the mailing list (ovirt users or
> devel), so other will be able to both chime in and see the response.
> We've supported a custom hook for enabling discard per disk (which is only
> relevant for virtio-SCSI and IDE) for some versions now (3.5 I believe).
> We are planning to add this via a UI and API in 4.1.
> In addition, we are looking into discard (instead of wipe after delete,
> when discard is also zero'ing content) as well as discard when removing LVs.
> See:
> http://www.ovirt.org/develop/release-management/features/
> storage/pass-discard-from-guest-to-underlying-storage/
> http://www.ovirt.org/develop/release-management/features/
> storage/wipe-volumes-using-blkdiscard/
> http://www.ovirt.org/develop/release-management/features/
> storage/discard-after-delete/
> Y.
>> Best,
>> --
>> Nicolas ECARNOT
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