On 04.11.19 16:12, Alberto Garcia wrote: > On Mon 04 Nov 2019 03:25:12 PM CET, Max Reitz wrote: >>>>>> So, it's obvious that c8bb23cbdbe32f5c326 is significant for 1M >>>>>> cluster-size, even on rotational disk, which means that previous >>>>>> assumption about calling handle_alloc_space() only for ssd is wrong, >>>>>> we need smarter heuristics.. >>>>>> >>>>>> So, I'd prefer (1) or (2). >>>> >>>> OK. I wonder whether that problem would go away with Berto’s subcluster >>>> series, though. >>> >>> Catching up with this now. I was told about this last week at the KVM >>> Forum, but if the problems comes with the use of fallocate() and XFS, >>> the I don't think subclusters will solve it. >>> >>> handle_alloc_space() is used to fill a cluster with zeroes when there's >>> COW, and that happens the same with subclusters, just at the subcluster >>> level instead of course. >>> >>> What can happen, if the subcluster size matches the filesystem block >>> size, is that there's no need for any COW and therefore the bug is never >>> triggered. But that's not quite the same as a fix :-) >> >> No, what I meant was that the original problem that led to c8bb23cbdbe >> would go away. > > Ah, right. Not quite, according to my numbers: > > |--------------+----------------+-----------------+-------------| > | Cluster size | subclusters=on | subclusters=off | fallocate() | > |--------------+----------------+-----------------+-------------| > | 256 KB | 10182 IOPS | 966 IOPS | 14007 IOPS | > | 512 KB | 7919 IOPS | 563 IOPS | 13442 IOPS | > | 1024 KB | 5050 IOPS | 465 IOPS | 13887 IOPS | > | 2048 KB | 2465 IOPS | 271 IOPS | 13885 IOPS | > |--------------+----------------+-----------------+-------------| > > There's obviously no backing image, and only the last column uses > handle_alloc_space() / fallocate().
Thanks for providing some numbers! It was my impression, too, that subclusters wouldn’t solve it. But it didn’t seem like that big of a difference to me. Did you run this with aio=native? (Because that’s where we have the XFS problem) Max
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