<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Mon, Feb 12, 2018 at 1:55 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 Idan,<br>
<br>
thanks for your answer. But i'm still confused, because i thought that the content of /sys/block/dm-X/queue/discard* in the VM OS should depend on the setting of the "discard=(unmap|ignore)" setting in the qemu-kvm command. Unexpectedly it's the same in both cases (it's >0, saying discard is 'on'). I was then trying to inquire about the TRIM/UNMAP capability of block devices in the VM with "sdparm -p lbp /dev/sdx", but i always get "Logical block provisioning (SBC) mode subpage failed".<br></blockquote><div>The file /sys/block/dm-X/queue/discard_max_bytes in sysfs tells you whether your underlying storage supports discard.</div><div>The flag discard=unmap of the VM in qemu means that qemu will not throw away the UNMAP commands comming from the guest OS (by default it does throw them away).</div><div>From what I know, the file in sysfs and the VM flag are not related.<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<br>
I know i can (and should) look at the storage array to see if TRIM/UNMAP _actually_ works, but documentation (<a href="https://www.kernel.org/doc/Documentation/block/queue-sysfs.txt" rel="noreferrer" target="_blank">https://www.kernel.org/doc/Do<wbr>cumentation/block/queue-sysfs.<wbr>txt</a>) says it should be visible beforehand to my understanding.<br>
<br>
Regards<br>
Matthias<span class="gmail-"><br>
<br>
Am 2018-02-11 um 08:55 schrieb Idan Shaby:<br>
</span><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><span class="gmail-">
Hi Matthias,<br>
<br>
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>
Therefore, no change is visible to the guest OS.<br>
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>
<br>
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>
If you got any further questions, please don't hesitate to ask.<br>
<br>
<br>
Regards,<br>
Idan<br>
<br>
[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/" rel="noreferrer" target="_blank">https://www.ovirt.org/develop/<wbr>release-management/features/st<wbr>orage/pass-discard-from-guest-<wbr>to-underlying-storage/</a><br>
<br></span><span class="gmail-">
On Thu, Feb 8, 2018 at 2:08 PM, Matthias Leopold <<a href="mailto:matthias.leopold@meduniwien.ac.at" target="_blank">matthias.leopold@meduniwien.a<wbr>c.at</a> <mailto:<a href="mailto:matthias.leopold@meduniwien.ac.at" target="_blank">matthias.leopold@medun<wbr>iwien.ac.at</a>>> wrote:<br>
<br>
Hi,<br>
<br>
i'm sorry to bother you again with my ignorance of the DISCARD<br>
feature for block devices in general.<br>
<br>
after finding several ways to enable "discard=unmap" for oVirt disks<br>
(via standard GUI option for iSCSI disks or via "diskunmap" custom<br>
property for Cinder disks) i wanted to check in the guest for the<br>
effectiveness of this feature. to my surprise i couldn't find a<br>
difference between Linux guests with and without "discard=unmap"<br>
enabled in the VM. "lsblk -D" reports the same in both cases and<br>
also fstrim/blkdiscard commands appear to work with no difference.<br>
Why is this? Do i have to look at the underlying storage to find out<br>
what really happens? Shouldn't this be visible in the guest OS?<br>
<br>
thx<br>
matthias<br>
<br>
______________________________<wbr>_________________<br>
Users mailing list<br></span>
<a href="mailto:Users@ovirt.org" target="_blank">Users@ovirt.org</a> <mailto:<a href="mailto:Users@ovirt.org" target="_blank">Users@ovirt.org</a>><br>
<a href="http://lists.ovirt.org/mailman/listinfo/users" rel="noreferrer" target="_blank">http://lists.ovirt.org/mailman<wbr>/listinfo/users</a><br>
<<a href="http://lists.ovirt.org/mailman/listinfo/users" rel="noreferrer" target="_blank">http://lists.ovirt.org/mailma<wbr>n/listinfo/users</a>><br>
<br>
<br><span class="gmail-HOEnZb"><font color="#888888">
</font></span></blockquote><span class="gmail-HOEnZb"><font color="#888888">
<br>
-- <br>
Matthias Leopold<br>
IT Systems & Communications<br>
Medizinische Universität Wien<br>
Spitalgasse 23 / BT 88 /Ebene 00<br>
A-1090 Wien<br>
Tel: <a href="tel:%2B43%201%2040160-21241" value="+4314016021241" target="_blank">+43 1 40160-21241</a><br>
Fax: <a href="tel:%2B43%201%2040160-921200" value="+43140160921200" target="_blank">+43 1 40160-921200</a><br>
</font></span></blockquote></div><br></div></div>