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

[Liu Yuan]

On Thu, Feb 12, 2015 at 11:33:16AM +0900, Teruaki Ishizaki wrote:
> (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 <address@hidden>
> >>>>---
> >>>>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?

I checked the QEMU code and got your idea. In the past it was fixed size so very
easy to hardcode the check in the client, no communication with sheep needed.

Yes, if it is reasonable, we can change it.

I think we can push the size calculation logic into sheep, if not the right size
return INVALID_PARAMETER to clients. Clients just check this and report error
back to users.

There is no backward compability for this approach, since 4MB is the smallest
size.

OLD QEMU will limit the max_size as 4TB, which is no problem for new sheep.

Thanks
Yuan