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

[Kevin Wolf]

Am 06.12.2013 um 18:55 hat Paolo Bonzini geschrieben:
> Il 06/12/2013 18:22, Kevin Wolf ha scritto:
> > This patch series adds code to the block layer that allows performing
> > I/O requests in smaller granularities than required by the host backend
> > (most importantly, O_DIRECT restrictions). It achieves this for reads
> > by rounding the request to host-side block boundary, and for writes by
> > performing a read-modify-write cycle (and serialising requests
> > touching the same block so that the RMW doesn't write back stale data).
> > 
> > Originally I intended to reuse a lot of code from Paolo's previous
> > patch series, however as I tried to integrate pread/pwrite, which
> > already do a very similar thing (except for considering concurrency),
> > and because I wanted to implement zero-copy, most of this series ended
> > up being new code.
> 
> Very nice!  There's not much to comment on regarding the 512-on-4k part.

Thanks. Then I guess it makes sense to push it before attacking
4k-on-512.

> Just one question.  Do you think we could make our emulated disks
> 512b-logical/4k-physical (512E) by default?  In other words, I don't
> remember the outcome of our discussion on whether the standard promises
> a 4k write granularity for this configuration.

It probably depends on our answer to the questions I raised for
4k-on-512. In generic, I think it's a good idea, but if we come to the
conclusion that it isn't safe, we can't do it, obviously.

As for the standard, which "the" standard? I guess virtio-blk doesn't
say anything on the matter, leaves us with ATA and SBC. I haven't
checked either yet, but I'll see if I can find something.

Kevin