[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.
Regards
Matthias
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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