Re: [Qemu-devel] [PATCH v4] sheepdog: selectable object size support

[Teruaki Ishizaki <ishizaki.teruaki@lab.ntt.co.jp>]

(2015/02/12 11:19), Liu Yuan wrote:
> On Thu, Feb 12, 2015 at 10:51:25AM +0900, Teruaki Ishizaki wrote:
>> (2015/02/10 20:12), Liu Yuan wrote:
>>> On Tue, Jan 27, 2015 at 05:35:27PM +0900, Teruaki Ishizaki wrote:
>>>> Previously, qemu block driver of sheepdog used hard-coded VDI object size.
>>>> This patch enables users to handle "block_size_shift" value for
>>>> calculating VDI object size.
>>>>
>>>> When you start qemu, you don't need to specify additional command option.
>>>>
>>>> But when you create the VDI which doesn't have default object size
>>>> with qemu-img command, you specify block_size_shift option.
>>>>
>>>> If you want to create a VDI of 8MB(1 << 23) object size,
>>>> you need to specify following command option.
>>>>
>>>>   # qemu-img create -o block_size_shift=23 sheepdog:test1 100M
>>>>
>>>> In addition, when you don't specify qemu-img command option,
>>>> a default value of sheepdog cluster is used for creating VDI.
>>>>
>>>>   # qemu-img create sheepdog:test2 100M
>>>>
>>>> Signed-off-by: Teruaki Ishizaki <ishizaki.teruaki@lab.ntt.co.jp>
>>>> ---
>>>> V4:
>>>>   - Limit a read/write buffer size for creating a preallocated VDI.
>>>>   - Replace a parse function for the block_size_shift option.
>>>>   - Fix an error message.
>>>>
>>>> V3:
>>>>   - Delete the needless operation of buffer.
>>>>   - Delete the needless operations of request header.
>>>>     for SD_OP_GET_CLUSTER_DEFAULT.
>>>>   - Fix coding style problems.
>>>>
>>>> V2:
>>>>   - Fix coding style problem (white space).
>>>>   - Add members, store_policy and block_size_shift to struct SheepdogVdiReq.
>>>>   - Initialize request header to use block_size_shift specified by user.
>>>> ---
>>>>   block/sheepdog.c          |  138 ++++++++++++++++++++++++++++++++++++++-------
>>>>   include/block/block_int.h |    1 +
>>>>   2 files changed, 119 insertions(+), 20 deletions(-)
>>>>
>>>> diff --git a/block/sheepdog.c b/block/sheepdog.c
>>>> index be3176f..a43b947 100644
>>>> --- a/block/sheepdog.c
>>>> +++ b/block/sheepdog.c
>>>> @@ -37,6 +37,7 @@
>>>>   #define SD_OP_READ_VDIS      0x15
>>>>   #define SD_OP_FLUSH_VDI      0x16
>>>>   #define SD_OP_DEL_VDI        0x17
>>>> +#define SD_OP_GET_CLUSTER_DEFAULT   0x18
>>>
>>> This might not be necessary. For old qemu or the qemu-img without setting
>>> option, the block_size_shift will be 0.
>>>
>>> If we make 0 to represent 4MB object, then we don't need to get the default
>>> cluster object size.
>>>
>>> We migth even get rid of the idea of cluster default size. The downsize is that,
>>> if we want to create a vdi with different size not the default 4MB,
>>> we have to write it every time for qemu-img or dog.
>>>
>>> If we choose to keep the idea of cluster default size, I think we'd also try to
>>> avoid call this request from QEMU to make backward compatibility easier. In this
>>> scenario, 0 might be used to ask new sheep to decide to use cluster default size.
>>>
>>> Both old qemu and new QEMU will send 0 to sheep and both old and new sheep can
>>> handle 0 though it has different meanings.
>>>
>>> Table for this bit as 0:
>>> Qe: qemu
>>> SD: Sheep daemon
>>> CDS: Cluster Default Size
>>> Ign: Ignored by the sheep daemon
>>>
>>> Qe/sd   new    old
>>> new     CDS    Ign
>>> old     CDS    NULL
>> Does Ign mean that VDI is handled as 4MB object size?
>
> Yes, old sheep can only handle 4MB object and doesn't check this field at all.
>
>>
>>>
>>> I think this approach is acceptable. The difference to your patch is that
>>> we don't send SD_OP_GET_CLUSTER_DEFAULT to sheep daemon and
>>> SD_OP_GET_CLUSTER_DEFAULT can be removed.
>> When users create a new VDI with qemu-img, qemu's Sheepdog backend
>> driver calculates max limit VDI size.
>
>> But if block_size_shift option is not specified, qemu's Sheepdog backend
>> driver can't calculate max limit VDI size.
>
> If block_size_shift not specified, this means
>
> 1 for old sheep, use 4MB size
> 2 for new sheep, use cluster wide default value.
>
> And sheep then can calculate it on its own, no?
>
Dog command(client) calculate max size, so I think
that qemu's Sheepdog backend driver should calculate it
like dog command.

Is that policy changeable?
Is there no policy?

Thanks,
Teruaki