Re: [Libguestfs] [RFC] lib: allow to specify physical/logical block size for disks
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(a)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=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.
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