[ovirt-users] effectiveness of "discard=unmap"

Matthias Leopold matthias.leopold at meduniwien.ac.at
Mon Feb 12 11:55:50 UTC 2018

Hi Idan,

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".

I know i can (and should) look at the storage array to see if 
TRIM/UNMAP _actually_ works, but documentation 
(https://www.kernel.org/doc/Documentation/block/queue-sysfs.txt) says it 
should be visible beforehand to my understanding.


Am 2018-02-11 um 08:55 schrieb Idan Shaby:
> Hi Matthias,
> 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.
> Therefore, no change is visible to the guest OS.
> 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.
> 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).
> If you got any further questions, please don't hesitate to ask.
> Regards,
> Idan
> [1] Pass discard from guest to underlying storage - 
> https://www.ovirt.org/develop/release-management/features/storage/pass-discard-from-guest-to-underlying-storage/
> On Thu, Feb 8, 2018 at 2:08 PM, Matthias Leopold 
> <matthias.leopold at meduniwien.ac.at 
> <mailto:matthias.leopold at meduniwien.ac.at>> wrote:
>     Hi,
>     i'm sorry to bother you again with my ignorance of the DISCARD
>     feature for block devices in general.
>     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?
>     thx
>     matthias
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Matthias Leopold
IT Systems & Communications
Medizinische Universität Wien
Spitalgasse 23 / BT 88 /Ebene 00
A-1090 Wien
Tel: +43 1 40160-21241
Fax: +43 1 40160-921200

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