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

пн, 10 лют. 2020 о 15:48 Kevin Wolf <kwolf(a)redhat.com&gt; пише: > > 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 > So, can we summarize? - in libguestfs we will expose the only parameter called 'blocksize' - 512 and 4096 are the only allowed values for 'blocksize' for now

- we will reject libvirt XML with values for physical_* and logical_block_size other then 512 or 4096 - importing disks configuration from libvirt XML we will use logical_block_size Richard, are we fine with that?