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
