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