Re: [Libguestfs] [PATCH libnbd] copy: Implement destination preferred block size

[NB: I think this is a failed attempt, so shoudn't go upstream, and doesn't need to be reviewed.] When nbdcopy writes to an NBD server it ignores the server's minimum/preferred block size. This hasn't caused a problem til now, but it turns out that in at least one case we care about (writing to qemu-nbd backed by a compressed qcow2 file) we must obey the minimum & preferred block size of 64K. For the longer story on that see this thread: https://lists.nongnu.org/archive/html/qemu-devel/2022-01/threads.html#06108 This patch attempts to fix this. The uncontroversial part of this patch adds a global "blocksize" variable which is the destination preferred block size. The tricky part of this patch tries to ensure that writes to the destination obey this block size constraint. Since nbdcopy is driven by the extent map read from the source, the theory behind this implementation is that we read the extent map and then try to "adjust" it so that extents which are not aligned to the block size grow, shrink or are deleted. It proved to be very difficult to get that right, but you can see the implementation in the new function "adjust_extents_for_blocksize". Unfortunately not only is this difficult to implement, but the theory is wrong. Read requests are actually split up into smaller subrequests on write (look for "copy_subcommand" in multi-thread-copying.c). So this doesn't really solve the problem.

So I think in my second version I will look at adjusting the NBD destination driver (nbd-ops.c) directly so that it turns unaligned writes into buffered read/modify/write operations (which can be optimized quite a lot because we understand the write pattern and know that the main program doesn't go backwards within blocks).