From: "Richard W.M. Jones" <rjones@redhat.com> To: Eric Blake <eblake@redhat.com> Cc: libguestfs@redhat.com, QEMU <qemu-devel@nongnu.org>, "qemu-block@nongnu.org" <qemu-block@nongnu.org>, nbd@other.debian.org Subject: Re: [Qemu-devel] [Libguestfs] cross-project patches: Add NBD Fast Zero support Date: Tue, 27 Aug 2019 13:14:49 +0100 Message-ID: <20190827121449.GX7304@redhat.com> (raw) In-Reply-To: <25ead363-4f37-5450-b985-1876374e314d@redhat.com> 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
next prev parent reply index Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-08-23 14:30 [Qemu-devel] " Eric Blake 2019-08-23 14:34 ` [Qemu-devel] [PATCH 0/1] NBD protocol change to add fast zero support Eric Blake 2019-08-23 14:34 ` [Qemu-devel] [PATCH 1/1] protocol: Add NBD_CMD_FLAG_FAST_ZERO Eric Blake 2019-08-23 18:48 ` Wouter Verhelst 2019-08-23 18:58 ` Eric Blake 2019-08-24 6:44 ` Wouter Verhelst 2019-08-28 9:57 ` Vladimir Sementsov-Ogievskiy 2019-08-28 13:04 ` Eric Blake 2019-08-28 13:45 ` Vladimir Sementsov-Ogievskiy 2019-09-03 20:53 ` [Qemu-devel] [Libguestfs] [PATCH 0/1] NBD protocol change to add fast zero support Eric Blake 2019-08-23 14:37 ` [Qemu-devel] [PATCH 0/5] Add NBD fast zero support to qemu client and server Eric Blake 2019-08-23 14:37 ` [Qemu-devel] [PATCH 1/5] nbd: Improve per-export flag handling in server Eric Blake 2019-08-30 18:00 ` Vladimir Sementsov-Ogievskiy 2019-08-30 23:10 ` Eric Blake 2019-08-30 23:32 ` Eric Blake 2019-09-03 16:39 ` Eric Blake 2019-09-04 17:08 ` Vladimir Sementsov-Ogievskiy 2019-08-23 14:37 ` [Qemu-devel] [PATCH 2/5] nbd: Prepare for NBD_CMD_FLAG_FAST_ZERO Eric Blake 2019-08-30 18:07 ` Vladimir Sementsov-Ogievskiy 2019-08-30 23:37 ` Eric Blake 2019-08-31 8:11 ` Vladimir Sementsov-Ogievskiy 2019-09-03 18:49 ` Eric Blake 2019-08-31 8:20 ` Vladimir Sementsov-Ogievskiy 2019-08-23 14:37 ` [Qemu-devel] [PATCH 3/5] nbd: Implement client use of NBD FAST_ZERO Eric Blake 2019-08-30 18:11 ` Vladimir Sementsov-Ogievskiy 2019-08-23 14:37 ` [Qemu-devel] [PATCH 4/5] nbd: Implement server " Eric Blake 2019-08-30 18:40 ` Vladimir Sementsov-Ogievskiy 2019-08-23 14:37 ` [Qemu-devel] [PATCH 5/5] nbd: Tolerate more errors to structured reply request Eric Blake 2019-08-23 16:41 ` Eric Blake 2019-08-28 13:55 ` [Qemu-devel] [PATCH 0/5] Add NBD fast zero support to qemu client and server Vladimir Sementsov-Ogievskiy 2019-08-28 14:05 ` Eric Blake 2019-08-23 14:38 ` [Qemu-devel] [libnbd PATCH 0/1] libnbd support for new fast zero Eric Blake 2019-08-23 14:38 ` [Qemu-devel] [libnbd PATCH 1/1] api: Add support for FAST_ZERO flag Eric Blake 2019-08-27 12:25 ` [Qemu-devel] [Libguestfs] " Richard W.M. Jones 2019-08-23 14:40 ` [Qemu-devel] [nbdkit PATCH 0/3] nbdkit support for new NBD fast zero Eric Blake 2019-08-23 14:40 ` [Qemu-devel] [nbdkit PATCH 1/3] server: Add internal support for NBDKIT_FLAG_FAST_ZERO Eric Blake 2019-08-23 14:40 ` [Qemu-devel] [nbdkit PATCH 2/3] filters: Add .can_fast_zero hook Eric Blake 2019-08-23 14:40 ` [Qemu-devel] [nbdkit PATCH 3/3] plugins: " Eric Blake 2019-08-23 21:16 ` [Qemu-devel] [Libguestfs] " Eric Blake 2019-08-27 15:43 ` Richard W.M. Jones 2019-08-23 15:05 ` [Qemu-devel] cross-project patches: Add NBD Fast Zero support Vladimir Sementsov-Ogievskiy 2019-08-27 12:14 ` Richard W.M. Jones [this message] 2019-08-27 13:23 ` [Qemu-devel] [Libguestfs] " Eric Blake
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