Re: [Qemu-devel] [RFC PATCH 00/19] block: Support for 512b-on-4k emulation

[Stefan Hajnoczi]

On Fri, Dec 06, 2013 at 06:22:41PM +0100, Kevin Wolf wrote:
> This series does not cover 4k guests on a 512 byte host, and I'm not
> sure yet what to do with this case. Paolos series contained a patch to
> protect against "torn reads" (i.e. reads running in parallel with
> writes, which return old data for one half of a sector and new data for
> the other half) by serialising requests if the guest block size was
> greater than the host block size.
> 
> One problem with this approach is that it assumes that a single host
> block size even exists and can be compared against on the top level.
> Different backing files can be stored on different storage, though, with
> different block sizes.

As long as the backing file is read-only you won't get torn reads.