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&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. 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