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

[Liu Yuan <namei.unix@gmail.com>]

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

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.

Thanks
Yuan