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

[Kevin Wolf <kwolf@redhat.com>]

Am 10.02.2020 um 12:43 hat Richard W.M. Jones geschrieben:
> On Sat, Feb 08, 2020 at 01:25:28AM +0200, Mykola Ivanets wrote:
> > From: Nikolay Ivanets <stenavin@gmail.com>
> > 
> > I faced with situation where libguestfs cannot recognize partitions on a
> > disk image which was partitioned on a system with "4K native" sector
> > size support.
> 
> Do you have a small test case for this?
> 
> > In order to fix the issue we need to allow users to specify desired
> > physical and/or logical block size per drive basis.
> 
> It seems like physical_block_size / logical_block_size in qemu are
> completely undocumented.  However I did some experiments with patching
> libguestfs and examining the qemu and parted code.  Here are my
> observations:
> 
> (1) Setting only physical_block_size = 4096 seems to do nothing.

The guest sees the physical_block_size and can try to keep its requests
aligned as an optimisation. But it doesn't actually make a semantic
difference as to how the content of the disk is accessed.

> (2) Setting only logical_block_size = 4096 is explicitly rejected by
> virtio-scsi:
> 
> https://git.qemu.org/?p=qemu.git;a=blob;f=hw/scsi/scsi-disk.c;h=10d0794d60f196f177563aae00bed2181f5c1bb1;hb=HEAD#l2352
> 
> (A similar test exists for virtio-blk)
> 
> (3) Setting both physical_block_size = logical_block_size = 4096
> changes how parted partitions GPT disks.  The partition table is
> clearly using 4K sectors as you can see by examining the disk
> afterwards with hexdump.

This is what you want for emulating a 4k native disk.

> (4) Neither setting changes MBR partitioning by parted, although my
> interpretation of Wikipedia indicates that it should be possible to
> create a MBR disk with 4K sector size.  Maybe I'm doing something
> wrong, or parted just doesn't support this case.

I seem to remember that 4k native disks require GPT, but if you say you
read otherwise, I'm not 100% sure about this any more.

> So it appears that we should just have one blocksize control (maybe
> called "sectorsize"?) which sets both physical_block_size and
> logical_block_size to the same value.  It may also be worth enforcing
> that blocksize/sectorsize must be set to 512 or 4096 (which we can
> relax later if necessary).

A single option (to control logical_block_size) makes sense for
libguestfs. physical_block_size is only relevant for the appliance and
not for the resulting image, so it can be treated as an implementation
detail.

Kevin