[Libguestfs] [RFC] lib: allow to specify physical/logical block size for disks

[Nikolay Ivanets]

пн, 10 лют. 2020 о 14:59 Richard W.M. Jones <rjones at redhat.com> пише:
>
> On Mon, Feb 10, 2020 at 02:22:01PM +0200, Nikolay Ivanets wrote:
> > Here are what they do stand for:
> > physical_block_size: The physical block size the disk will report to
> > the guest OS. For Linux this would be the value returned by the
> > BLKPBSZGET ioctl and describes the disk's hardware sector size which
> > can be relevant for the alignment of disk data. We don't have an API
> > to get this one.
> >
> > logical_block_size: The logical block size the disk will report to the
> > guest OS. For Linux this would be the value returned by the BLKSSZGET
> > ioctl and describes the smallest units for disk I/O. We have
> > blockdev-getsz API to get this value.
>
> Interestingly parted uses BLKSSZGET (logical_block_size), but does not
> use BLKPBSZGET (physical_block_size), so I guess that fits with my
> observations.
>
> > How do they use. If your HDD has physical block size = 4096 you might
> > want make I/O request equals to (or multiple) this value. Otherwise
> > you might hit performance penalty. I think, the same is valid for
> > virtual disk image which is located on physical storage with 4K
> > physical sector size.
> ...
> > Here are possible combinations of [pl]blocksize in real world:
> >
> > physical | logical
> > -----------------------
> > 4096      | 4096
> > 4096      | 512
> > 512        | 512
> >
> > Having both values equals works except for case #2 which will not
> > impact on functionality, only performance might be hit.
> > If we want to simplify our API - we might expose the only parameter
> > called 'blocksize' and set physical_block_size and logical_block_size
> > to the same value.
>
> Right, but libguestfs rarely deals with real physical disks.  I think
> even if eg. you attach libguestfs to /dev/sdX there's still going to
> be a host kernel in between doing its thing.  So I'm not clear that
> the 4K physical / 512b logical case is actually something that
> libguestfs cares about.  (But OTOH I'm also not clear why qemu cares
> about this case either.)

I think because QEMU emulates real hardware. And on real hardware you
can get different physical/logical block size. So QEMU allows you to create
such a configuration.

--
  Mykola Ivanets