Re: [Qemu-discuss] Block format 'qcow2' used by device 'drive-virtio0' doesn't support the option 'physical_block_size'

[Jakob Bohm]

On 27/01/2015 17:52, Hendrik Visage wrote:
> Good day,
>
>   Anybody able to guide me the combinations, etc. when and where these
> are supported?
> I'm having performance issues on 512e advance format/4Kb disks, and
> the symptoms for me tells me that I'm hitting the read-modify-write
> problems as the VMs report 512/512 for the virtio block devices, and I
> want to modify that to 4k sector sizes.
>
> Hendrik Visage
qcow2 cannot store the "physical block size" as an explicit
property.  But what you can do is the following:

1. Make sure the host physical disk partition system that stores
the qcow2 file is aligned to a multiple of 4K (or the RAID block
size if on a RAID system).

2. Make sure the host file system that stores the qcow2 file has
a block size of 4K or a multiple of 4K.

3. Make sure the internal qcow2 cluster_size is 4K or a multiple
of 4K (I think this is the default).  Otherwise this is set using
the "-o" "cluster_size=4096" option to qemu-img create/convert.

4. Make sure the guest partition on the virtual disk (backed by
the qcow2 file) is aligned on a multiple of the qcow2
cluster_size.

5. Make sure the guest file system of the guest partition on the
virtual disk has a block size which is a multiple of the qcow2
cluster_size.

In other words, the usual "4K issue" procedures, but on both the
physical and virtual machine.


Enjoy

Jakob
--
Jakob Bohm, CIO, Partner, WiseMo A/S.  http://www.wisemo.com
Transformervej 29, 2860 Søborg, Denmark.  Direct +45 31 13 16 10
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