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

[Matthias Leopold]

Am 2018-02-12 um 13:40 schrieb Idan Shaby:
> On Mon, Feb 12, 2018 at 1:55 PM, Matthias Leopold 
> <matthias.leopold at meduniwien.ac.at 
> <mailto:matthias.leopold at 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