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-disca...

On Thu, Feb 8, 2018 at 2:08 PM, Matthias Leopold <matthias.leopold@meduniwien.ac.at <mailto:matthias.leopold@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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Am 2018-02-12 um 13:40 schrieb Idan Shaby:

On Mon, Feb 12, 2018 at 1:55 PM, Matthias Leopold <matthias.leopold@meduniwien.ac.at <mailto:matthias.leopold@meduniwien.ac.at>> wrote:

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".

The file /sys/block/dm-X/queue/discard_max_bytes in sysfs tells you whether your underlying storage supports discard. 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). From what I know, the file in sysfs and the VM flag are not related.

Thank you, i will now finally accept it ;-) Just for the records: This is where i got my info from: https://chrisirwin.ca/posts/discard-with-kvm/ Matthias