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Date: Tue, 08 Sep 2015 14:02:05 -0700
From: Josh Durgin <jdurgin@redhat.com>
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Subject: Re: [Qemu-devel] where can i customize rbd object size?
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To: Stefan Hajnoczi <stefanha@gmail.com>, Jaze Lee <jazeltq@gmail.com>
Cc: qemu-devel@nongnu.org

On 09/03/2015 09:48 AM, Stefan Hajnoczi wrote:
> On Wed, Aug 19, 2015 at 03:39:20PM +0800, Jaze Lee wrote:
>> Hello,
>>
>>      qemu-img convert -f qcow2 Trove---mysql-5.6---2015-07-16.qcow2 -O raw
>> rbd:openstack-00/8205d01a-874c-44c0-b114-1c03821fcc24:conf=/etc/ceph/ceph.conf
>>
>>     How can i specify the object size that rbd uses?  I found that the
>> qemu-image can only use the default object size. It is defined in
>> block/rbd.c
>>
>>      #define OBJ_MAX_SIZE
>> <https://lxr.missinglinkelectronics.com/qemu+v2.1.0/+code=OBJ_MAX_SIZE>
>> (1UL << OBJ_DEFAULT_OBJ_ORDER
>> <https://lxr.missinglinkelectronics.com/qemu+v2.1.0/+code=OBJ_DEFAULT_OBJ_ORDER>
>> )

Those are remnants from before librbd existed, they aren't used in qemu
anymore.

>>      If someone know how to specify the object size, please tell me. thanks
>> a lot.
>>      By the way i do not find the define for  OBJ_DEFAULT_OBJ_ORDER
>> <https://lxr.missinglinkelectronics.com/qemu+v2.1.0/+code=OBJ_DEFAULT_OBJ_ORDER>,
>> if someone knows , please tell me. Thanks a lot.
>
> I have CCed the rbd.c maintainer for you:
> $ scripts/get_maintainer.pl -f block/rbd.c
> Josh Durgin <jdurgin@redhat.com> (supporter:RBD)
>

Thanks for CCing me, I missed it before.

You can specify the rbd object size in bytes via the cluster_size
option, e.g.

qemu-img convert -f qcow2 Trove---mysql-5.6---2015-07-16.qcow2 -O raw -o 
cluster_size=8388608 
rbd:openstack-00/8205d01a-874c-44c0-b114-1c03821fcc24:conf=/etc/ceph/ceph.conf

The code handling this is here:

https://lxr.missinglinkelectronics.com/#qemu+v2.1.0/block/rbd.c#L318

Josh