[ovirt-users] OVirt 4.1.2 - trim/discard on HDD/XFS/NFS contraproductive

[Markus Stockhausen]

> Von: Yaniv Kaul [ykaul at redhat.com]
> Gesendet: Sonntag, 18. Juni 2017 09:58
> An: Markus Stockhausen
> Cc: Ovirt Users
> Betreff: Re: [ovirt-users] OVirt 4.1.2 - trim/discard on HDD/XFS/NFS contraproductive

On Sat, Jun 17, 2017 at 1:25 AM, Markus Stockhausen <stockhausen at collogia.de<mailto:stockhausen at collogia.de>> wrote:
Hi,

we just set up a new 4.1.2 OVirt cluster. It is a quite normal
HDD/XFS/NFS stack that worked quit well with 4.0 in the past.
Inside the VMs we use XFS too.

To our surprise we observe abysmal high IO during mkfs.xfs
and fstrim inside the VM. A simple example:

Step 1: Create 100G Thin disk
Result 1: Disk occupies ~10M on storage

Step 2: Format disk inside VM with mkfs.xfs
Result 2: Disk occupies 100G on storage

Changing the discard flag on the disk does not have any effect.

> Are you sure it's discarding, at all?
> 1. NFS: only NFSv4.2 supports discard. Is that the case in your setup?
> 2. What's the value of /sys/block/<disk>/queue/discard_granularity ?
> 3. Can you share the mkfs.xfs command line?
> 4. Are you sure it's not a raw-sparse image?

Questions should answered in BZ1462504. When talking about thin
provisioned disks I'm only referring to the OVirt disk-option. So I
might mix up something here. Nevertheless the following is more than
strange for me:

- Create disk image: File on storage is small
- Format inside VM: File on storage is fully allocated
- Move around in Ovirt to another NFS storage: File is small again.

That means:
- mkfs.xfs inside VM and so qemu is hammering (empty) data into all blocks
- But this data must be zeros as they can be compated afterwards.

Best regards.

Markus

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