[ovirt-users] DISCARD support?

Nicolas Ecarnot nicolas at ecarnot.net
Tue Oct 4 08:11:28 UTC 2016


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.



On Mon, Oct 3, 2016 at 2:24 PM, Nicolas Ecarnot <nicolas at ecarnot.net 
<mailto:nicolas at ecarnot.net>> wrote:


    As a pure random way of web surfing, I found that you posted on
    twitter an information about DISCARD support.

    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 
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.



    Nicolas ECARNOT

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