Re: [Libguestfs] Supporting sparse disks in nbdkit

> int extents (void *handle, uint32_t count, uint64_t offset, > uint32_t flags /* always 0? */, Unless we support the REQ_ONE flag (where the client specifically wants only one extent returned).

> 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.) REQ_ONE says that a successful answer has to be exactly one extent, covering only the head of the region. (That is, force the server to behave as if obtaining information beyond the first extent is expensive). It also tells the server that it must not report extra information beyond the initial request (because at least qemu 3.0 would assert that the server supplied too much information). You're also right that nbdkit can do REQ_ONE massaging, so whether we pass REQ_ONE on to the client or not makes it sound like the client needs a tri-state opt-in (no block status support, block status but let nbdkit handle REQ_ONE, block status and the plugin can handle REQ_ONE efficiently itself)