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

[Idan Shaby]

Hi Markus,

AFAIK, mkfs.xfs tries to discard all the blocks before formatting the
device.
If you don't want it to do that, you can use the "-K     Do not attempt to
discard blocks at mkfs time" option of mkfs.xfs.

In oVirt 4.1 we introduced the "Enable Discard" flag for a virtual
machine's disk.
When enabled, qemu is configured to pass on live UNMAP SCSI commands from
the guest to the underlying storage.
If you don't need live discarding, shutdown the VM and disable the "Enable
Discard" option. That will cause qemu to ignore the live UNMAP SCSI
commands coming from the guest and not pass it on to the underlying storage.
Note that this makes fstrim completely redundant, as the purpose of the
command is to discard unused blocks under the given path.


Regards,
Idan

On Sat, Jun 17, 2017 at 1:25 AM, Markus Stockhausen <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.
>
> Am I missing something?
>
> Best regards.
>
> Markus
>
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