On Fri, Aug 23, 2019 at 09:30:36AM -0500, Eric Blake wrote:
I've run several tests to demonstrate why this is useful, as well as prove that because I have multiple interoperable projects, it is worth including in the NBD standard. The original proposal was here: https://lists.debian.org/nbd/2019/03/msg00004.html where I stated:
I will not push this without both: - a positive review (for example, we may decide that burning another NBD_FLAG_* is undesirable, and that we should instead have some sort of NBD_OPT_ handshake for determining when the server supports NBD_CMF_FLAG_FAST_ZERO) - a reference client and server implementation (probably both via qemu, since it was qemu that raised the problem in the first place)
Is the plan to wait until NBD_CMF_FLAG_FAST_ZERO gets into the NBD protocol doc before doing the rest? Also I would like to release both libnbd 1.0 and nbdkit 1.14 before we introduce any large new features. Both should be released this week, in fact maybe even today or tomorrow. [...]
First, I had to create a scenario where falling back to writes is noticeably slower than performing a zero operation, and where pre-zeroing also shows an effect. My choice: let's test 'qemu-img convert' on an image that is half-sparse (every other megabyte is a hole) to an in-memory nbd destination. Then I use a series of nbdkit filters to force the destination to behave in various manners: log logfile=>(sed ...|uniq -c) (track how many normal/fast zero requests the client makes) nozero $params (fine-tune how zero requests behave - the parameters zeromode and fastzeromode are the real drivers of my various tests) blocksize maxdata=256k (allows large zero requests, but forces large writes into smaller chunks, to magnify the effects of write delays and allow testing to provide obvious results with a smaller image) delay delay-write=20ms delay-zero=5ms (also to magnify the effects on a smaller image, with writes penalized more than zeroing) stats statsfile=/dev/stderr (to track overall time and a decent summary of how much I/O occurred). noextents (forces the entire image to report that it is allocated, which eliminates any testing variability based on whether qemu-img uses that to bypass a zeroing operation [1])
I can't help thinking that a sh plugin might have been simpler ...
I hope you enjoyed reading this far, and agree with my interpretation of the numbers about why this feature is useful!
Yes it seems reasonable. The only thought I had is whether the qemu block layer does or should combine requests in flight so that a write-zero (offset) followed by a write-data (same offset) would erase the earlier request. In some circumstances that might provide a performance improvement without needing any changes to protocols.
- NBD should have a way to advertise (probably via NBD_INFO_ during NBD_OPT_GO) if the initial image is known to begin life with all zeroes (if that is the case, qemu-img can skip the extents calls and pre-zeroing pass altogether)
Yes, I really think we should do this one as well. 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