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

From: Richard W.M. Jones
Subject: Re: [Libguestfs] [RFC] lib: allow to specify physical/logical block size for disks
Date: Mon, 10 Feb 2020 11:43:16 +0000
User-agent: Mutt/1.5.21 (2010-09-15)

On Sat, Feb 08, 2020 at 01:25:28AM +0200, Mykola Ivanets wrote:
> From: Nikolay Ivanets <address@hidden>
> 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

(1) Setting only physical_block_size = 4096 seems to do nothing.

(2) Setting only logical_block_size = 4096 is explicitly rejected by


(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.

(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.

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).


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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