Re: Questions about the CRUSH details

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On 1/25/24 13:32, Janne Johansson wrote:

It doesn't take OSD usage into consideration except at creation time
or OSD in/out/reweighing (or manual displacements with upmap and so
forth), so this is why "ceph df" will tell you a pool has X free
space, where X is "smallest free space on the OSDs on which this pool
lies, times the number of OSDs". Given the pseudorandom placement of
objects to PGs, there is nothing to prevent you from having the worst
luck ever and all the objects you create end up on the OSD with least
free space.

This is why you need a decent amount of PGs, to not run into statistical edge cases.

Robert Sander
Heinlein Consulting GmbH
Schwedter Str. 8/9b, 10119 Berlin

Tel: 030 / 405051-43
Fax: 030 / 405051-19

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