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

On Sat, Feb 08, 2020 at 01:25:28AM +0200, Mykola Ivanets wrote: > From: Nikolay Ivanets <stenavin(a)gmail.com&gt; > > 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.

(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=10d0794d6... (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).