Re: [Qemu-devel] QEMU interfaces for image streaming and post-copy block migration

[Anthony Liguori <aliguori@linux.vnet.ibm.com>]

On 09/07/2010 10:02 AM, Kevin Wolf wrote:
> Am 07.09.2010 16:49, schrieb Anthony Liguori:
>    
>>> Shouldn't it be a runtime option? You can use the very same image with
>>> copy-on-read or copy-on-write and it will behave the same (execpt for
>>> performance), so it's not an inherent feature of the image file.
>>>
>>>        
>> The way it's implemented in QED is that it's a compatible feature.  This
>> means that implementations are allowed to ignore it if they want to.
>> It's really a suggestion.
>>      
> Well, the point is that I see no reason why an image should contain this
> suggestion. There's really nothing about an image that could reasonably
> indicate "use this better with copy-on-read than with copy-on-write".
>
> It's a decision you make when using the image.
>    

Copy-on-read is, in many cases, a property of the backing file because 
it suggests that the backing file is either very slow or potentially 
volatile.

IOW, let's say I'm an image distributor and I want to provide my images 
in a QED format that actually streams the image from an http server.  I 
could provide a QED file without a copy-on-read bit set but I'd really 
like to convey this information as part of the image.

You can argue that I should provide a config file too that contained the 
copy-on-read flag set but you could make the same argument about backing 
files too.

>> So yes, you could have a run time switch that overrides the feature bit
>> on disk and either forces copy-on-read on or off.
>>
>> Do we have a way to pass block drivers run time options?
>>      
> We'll get them with -blockdev. Today we're using colons for format
> specific and separate -drive options for generic things.
>    

That's right.  I think I'd rather wait for -blockdev.

>> You need to understand the cluster boundaries in order to optimize the
>> metadata updates.  Sure, you can expose interfaces to the block layer to
>> give all of this info but that's solving the same problem for doing
>> block level copy-on-write.
>>
>> The other challenge is that for copy-on-read to be efficiently, you
>> really need a format that can distinguish between unallocated sectors
>> and zero sectors and do zero detection during the copy-on-read
>> operation.  Otherwise, if you have a 10G virtual disk with a backing
>> file that's 1GB is size, copy-on-read will result in the leaf being 10G
>> instead of ~1GB.
>>      
> That's a good point. But it's not a reason to make the interface
> specific to QED just because other formats would probably not implement
> it as efficiently.
>    

You really can't do as good of a job in the block layer because you have 
very little info about the characteristics of the disk image.

Regards,

Anthony Liguori

> Kevin
>