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

[Richard W.M. Jones]

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.)

Rich.

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