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

From: Teruaki Ishizaki
Subject: Re: [Qemu-devel] [PATCH v4] sheepdog: selectable object size support
Date: Fri, 06 Feb 2015 16:57:55 +0900
User-agent: Mozilla/5.0 (Windows NT 6.1; rv:24.0) Gecko/20100101 Thunderbird/24.6.0

(2015/02/06 11:18), Liu Yuan wrote:
On Wed, Feb 04, 2015 at 01:54:19PM +0900, Teruaki Ishizaki wrote:
(2015/02/02 15:52), 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

Is it possible to make this option more user friendly? such as

  $ qemu-img create -o object_size=8M sheepdog:test 1G

At first, I thought that the object_size was user friendly.
But, Sheepdog has already the value of block_size_shift
in the inode layout that means like object_size.

'object_size' doesn't always fit right in 'block_size_shift'.
On the other hands, 'block_size_shift' always fit right in

I think that existing layout shouldn't be changed easily and
it seems that it is difficult for users to specify
the object_size value that fit right in 'block_size_shift'.

I don't think we need to change the layout. block_size_shift is what QEMU talks
to sheep, and QEMU options is what users talks to QEMU. We can convert the user
friendly object size into block_size_shift internally in the driver before
sending it tosheep daemon, no?

For example, users specify 12MB for object size, block_size_shift
doesn't fit exactly to an integer.

I suppose that normally an administrator do format sheepdog cluster
with specifying block_size_shift and users usually do qemu-img command
without a block_size_shift option.

I think that the block_size_shift option of qemu-img command is used
for a experimental purpose.

Teruaki Ishizaki

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