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Date: Wed, 02 Oct 2013 17:13:44 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCHv4] block/get_block_status: avoid redundant
 callouts on raw devices
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: kwolf@redhat.com, anthony@codemonkey.ws, Peter Lieven <pl@kamp.de>, qemu-devel@nongnu.org, stefanha@redhat.com, ronniesahlberg@gmail.com

Il 02/10/2013 17:06, Stefan Hajnoczi ha scritto:
> Sorry I didn't review this earlier but this flag looks hacky and I'm not
> confident about merging the patch yet.
> 
> The patch makes me wonder if the raw_bsd driver should avoid calling
> bs->file itself:
> 
> return BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID |
>        (sector_num << BDRV_SECTOR_BITS);
> 
> Let block.c:bdrv_co_get_block_status() call down into bs->file.
> 
> The problem is then the protocol cannot report unallocated sectors with
> this approach.
> 
> I think we want to preserve bs' offset while taking the other flags from
> bs->file (DATA, ZERO).

This would cause other changes.  For example, a qcow2 with full metadata
preallocation (i.e. all L2 tables are there but it points to holes)
would not return DATA anymore.  I think this is wrong, and especially a
change from the old is_allocated API.

However, a variant on this idea could be to return

   BDRV_BLOCK_RAW | BDRV_BLOCK_OFFSET_VALID |
        (sector_num << BDRV_SECTOR_BITS);

and then BDRV_BLOCK_RAW would mean "take DATA and ZERO from bs->file".

Paolo

> Peter, Paolo: What do you think of this approach?