[Libguestfs] Supporting sparse disks in nbdkit

I've posted a couple of patches towards the ultimate goal of implementing NBD_CMD_BLOCK_STATUS / base:allocation in nbdkit. Before I can do the final patch I think we need to discuss how this would be exposed to plugins since at the end of the day they need to implement the feature. Background reading: - preparatory patches: https://www.redhat.com/archives/libguestfs/2019-March/msg00013.html https://www.redhat.com/archives/libguestfs/2019-March/msg00016.html - NBD protocol, see in particular NBD_CMD_BLOCK_STATUS and NBD_REPLY_TYPE_BLOCK_STATUS: https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md I think we shouldn't modify the pread() callback. If we decide to implement Structured Replies properly at some point in the future we might need to do that, but it's not necessary now. We could introduce a new call ‘extents’ to return the list of extents. I believe it would look like this: struct nbdkit_extent { uint64_t offset; uint32_t length; // XXX is 32 bit right here? uint32_t flag; // hole, zero, data ... more in future? }; int extents (void *handle, uint32_t count, uint64_t offset, uint32_t flags /* always 0? */, size_t *nr_extents, struct nbdkit_extent *extents); The function is meant to scan [offset, offset+count-1] and return a list of all extents overlapping this range, and their status (hole, zero, data). To make writing plugins easier we could say that extents don't need to be returned in order, and may include extents which don't actually overlap the requested range. Also missing regions would mean "hole" (makes writing the VDDK plugin easier), and adjacent extents of the same type would be coalesced automatically. But it's an error if returned extents overlap each other. nbdkit would need to do some massaging on this to get it into the right format for NBD_CMD_BLOCK_STATUS. (I'm very confused about what NBD_CMD_FLAG_REQ_ONE is supposed to do.) We will also need a corresponding ‘can_extents’, which is analogous to ‘can_write’ etc and is what would control the output of NBD_OPT_{SET,LIST}_META_CONTEXT. For nbdkit-file-filter: - Fairly simple implementation using SEEK_HOLE/SEEK_DATA. - Not sure how we detect zeroes without reading the file. For nbdkit-memory-plugin: - Pretty simple implementation, which can even detect non-hole zeroes. For VDDK: - VixDiskLib_QueryAllocatedBlocks can return allocated blocks, but doesn't return holes separately (they are assumed from what is omitted from the list). No support for detecting zeroes that I can see. Some existing filters would have to be modified to correctly adjust ‘extents’ offsets: - nbdkit-offset-filter - nbdkit-partition-filter - nbdkit-truncate-filter (? maybe not) - nbdkit-xz-filter is complicated: XZ files support sparseness so in theory we should try to return this data Your thoughts on this appreciated, Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://people.redhat.com/~rjones/virt-top