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

[Matthias Leopold]

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