[Libguestfs] Kernel driver I/O block size hinting

This is a follow-up to this thread: https://listman.redhat.com/archives/libguestfs/2022-June/thread.html#29210 about getting the kernel client (nbd.ko) to obey block size constraints sent by the NBD server: https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md#block-... I was sent this very interesting design document about the original intent behind the kernel's I/O limits: https://people.redhat.com/msnitzer/docs/io-limits.txt There are four or five kernel block layer settings we could usefully adjust, and there are three NBD block size constraints, and in my opinion there's not a very clear mapping between them. But I'll have a go at what I think we should do. - - - (1) Kernel physical_block_size & logical_block_size: The example given is of a hard disk with 4K physical sectors (AF) which can nevertheless emulate 512-byte sectors. In this case you'd set physical_block_size = 4K, logical_block_size = 512b. Data structures (partition tables, etc) should be aligned to physical_block_size to avoid unnecessary RMW cycles. But the fundamental until of I/O is logical_block_size. Current behaviour of nbd.ko is that logical_block_size == physical_block_size == the nbd-client "-b" option (default: 512 bytes, contradicting the documentation). I think we should set logical_block_size == physical_block_size == MAX (512, NBD minimum block size constraint). What should happen to the nbd-client -b option? (2) Kernel minimum_io_size: The documentation says this is the "preferred minimum unit for random I/O". Current behaviour of nbd.ko is this is not set. I think the NBD's preferred block size should map to minimum_io_size. (3) Kernel optimal_io_size: The documentation says this is the "[preferred] streaming I/O [size]". Current behaviour of nbd.ko is this is not set. NBD doesn't really have the concept of streaming vs random I/O, so we could either ignore this or set it to the same value as minimum_io_size. I have a kernel patch allowing nbd-client to set both minimum_io_size and optimal_io_size from userspace. (4) Kernel blk_queue_max_hw_sectors: This is documented as: "set max sectors for a request ... Enables a low level driver to set a hard upper limit, max_hw_sectors, on the size of requests." Current behaviour of nbd.ko is that we set this to 65536 (sectors? blocks?), which for 512b sectors is 32M. I think we could set this to MIN (32M, NBD maximum block size constraint), converting the result to sectors. - - - What do people think? Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-builder quickly builds VMs from scratch http://libguestfs.org/virt-builder.1.html