<div dir="ltr"><div><div><div><div>Hi Matthias,<br><br></div>When the guest executes a discard call of any variation (fstrim, blkdiscard, etc.), the underlying thinly provisioned LUN is the one that changes - it returns the unused blocks to the storage array and gets smaller.<br></div>Therefore, no change is visible to the guest OS.</div><div>If you want to check what has changed, go to the storage array and check what's the size of the underlying thinly provisioned LUN before and after the discard call.<br></div><br></div><div>The answer for your question and some more information can be found in the feature page [1] (needs a bit of an update, but most of it is still relevant).<br></div>If you got any further questions, please don't hesitate to ask.<br><div class="gmail_extra"><br clear="all"><div><div class="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><div><br>Regards,<br></div>Idan</div><div dir="ltr"><br></div><div dir="ltr">[1] Pass discard from guest to underlying storage - <a href="https://www.ovirt.org/develop/release-management/features/storage/pass-discard-from-guest-to-underlying-storage/">https://www.ovirt.org/develop/release-management/features/storage/pass-discard-from-guest-to-underlying-storage/</a><br></div></div></div></div></div></div></div>
<br><div class="gmail_quote">On Thu, Feb 8, 2018 at 2:08 PM, Matthias Leopold <span dir="ltr"><<a href="mailto:matthias.leopold@meduniwien.ac.at" target="_blank">matthias.leopold@meduniwien.ac.at</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi,<br>
<br>
i'm sorry to bother you again with my ignorance of the DISCARD feature for block devices in general.<br>
<br>
after finding several ways to enable "discard=unmap" for oVirt disks (via standard GUI option for iSCSI disks or via "diskunmap" custom property for Cinder disks) i wanted to check in the guest for the effectiveness of this feature. to my surprise i couldn't find a difference between Linux guests with and without "discard=unmap" enabled in the VM. "lsblk -D" reports the same in both cases and also fstrim/blkdiscard commands appear to work with no difference. Why is this? Do i have to look at the underlying storage to find out what really happens? Shouldn't this be visible in the guest OS?<br>
<br>
thx<br>
matthias<br>
<br>
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