Re: [Libguestfs] [PATCH v4 0/3] v2v: Add -o rhv-upload output mode.

On Mon, Mar 12, 2018 at 07:13:52AM +0000, Nir Soffer wrote: Yes. The 5 functions we could support are: * pread - done * pwrite - done * flush - does fdatasync(2) on the block device * zero - write a range of zeroes without having to send zeroes * trim - punch hole, can be emulated using zero if not possile Also (not implemented in nbdkit today, but coming soon), pwrite, zero and trim can be extended with a FUA (force unit access) flag, which would mean that the range should be persisted to disk before returning. It can be emulated by calling flush after the operation. It wasn't clear if anything in this process flushes the content to disk. Is that what transfer.finalize does? I think this depends on BLKDISCARDZEROES[1] for the block device? Of course if you're worried about data remanence for someone who has access to the physical device then that wouldn't be enough. [1] https://rwmj.wordpress.com/2014/03/11/blkdiscard-blkzeroout-blkdiscardzer... It depends a bit on what you mean by "aligned" and what the alignment is. We'd probably have to work around it in the plugin so that it can round in the request, issues a zero operation for the aligned part, and writes zeroes at each end. There's no guarantee that qemu-img will be well-behaved in the future even if it is now. Anyway this sounds do-able, is it something we can get for 4.2? How will we detect that the server supports it? Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-df lists disk usage of guests without needing to install any software inside the virtual machine. Supports Linux and Windows. http://people.redhat.com/~rjones/virt-df/