[Qemu-devel] Re: [PATCH 1/3] block: allow migration to work with image files

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

On 09/13/2010 03:21 AM, Kevin Wolf wrote:
> Am 11.09.2010 16:04, schrieb Anthony Liguori:
>    
>> Image files have two types of data: immutable data that describes things like
>> image size, backing files, etc. and mutable data that includes offset and
>> reference count tables.
>>
>> Today, image formats aggressively cache mutable data to improve performance.  In
>> some cases, this happens before a guest even starts.  When dealing with live
>> migration, since a file is open on two machines, the caching of meta data can
>> lead to data corruption.
>>
>> This patch addresses this by introducing a mechanism to invalidate any cached
>> mutable data a block driver may have which is then used by the live migration
>> code.
>>
>> NB, this still requires coherent shared storage.  Addressing migration without
>> coherent shared storage (i.e. NFS) requires additional work.
>>
>> Signed-off-by: Anthony Liguori<aliguori@us.ibm.com>
>>
>> diff --git a/block.c b/block.c
>> index ebbc376..cd2ee31 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -1453,6 +1453,22 @@ const char *bdrv_get_device_name(BlockDriverState *bs)
>>       return bs->device_name;
>>   }
>>
>> +void bdrv_invalidate_cache(BlockDriverState *bs)
>> +{
>> +    if (bs->drv&&  bs->drv->bdrv_invalidate_cache) {
>> +        bs->drv->bdrv_invalidate_cache(bs);
>> +    }
>> +}
>>      
> There is a simple generic implementation:
>
> drv = bs->drv;
> drv->close(bs);
> drv->open(bs, bs->open_flags);
>
> Note that this only reopens e.g. the qcow2 layer, but not the image
> file, which is bs->file.
>    

That's a pretty good idea for a general implementation, I'll update the 
patches accordingly.

Regards,

Anthony Liguori

> This works for all simple case, that is, one format on top of one or
> more protocols, where the protocols don't need invalidation. I think
> this includes everything that is possible today. With -blockdev we might
> need to revise this to include the lower layers, too. (But only
> sometimes, because we don't want to reopen the file)
>
> Kevin
>