Thanks Paul for the explanation, sounds very logical now. -----Original Message----- From: Paul Emmerich [mailto:paul.emmerich@xxxxxxxx] Sent: woensdag 25 april 2018 20:28 To: Marc Roos Cc: ceph-users Subject: Re: ceph osd reweight (doing -1 or actually -0.0001) Hi, the reweight is internally a number between 0 and 0x10000 for the range 0 to 1. 0.8 is not representable in this number system. Having an actual floating point number in there would be annoying because CRUSH needs to be 100% deterministic on all clients (also, no floating point in the kernel). osd reweight apparently just echoes whatever you entered here if it can't be mapped to a whole number: (Or it rounds differently, not sure and doesn't matter.) reweight 0.8 --> 0.8 * 0x10000 = 52428.8; is cut off to 52428 == 0xcccc. So it just prints out 0.8 but stores 0xcccc. The logic to read 0xcccc back and convert it back to the 0 - 1 range is: 0xcccc / 0x10000 = 0.79998779296 which is printed as 0.79999. Anyways, the important value is what is actually stored and that's 0xcccc. It could be argued that "osd reweight" should convert 0.8 to 0xcccd (0.80000305175), i.e. to round instead of cut off. Paul 2018-04-25 12:33 GMT+02:00 Marc Roos <M.Roos@xxxxxxxxxxxxxxxxx>: Makes me also wonder what is actually being used by ceph? And thus which one is wrong 'ceph osd reweight' output or 'ceph osd df' output. -----Original Message----- From: Marc Roos Sent: woensdag 25 april 2018 11:58 To: ceph-users Subject: ceph osd reweight (doing -1 or actually -0.0001) Is there some logic behind why ceph is doing this -1, or is this some coding error? 0.80000 gives 0.79999, and 0.80001 gives 0.80000 (ceph 12.2.4) [@~]# ceph osd reweight 11 0.80000 reweighted osd.11 to 0.8 (cccc) [@~]# ceph osd df ID CLASS WEIGHT REWEIGHT SIZE USE AVAIL %USE VAR PGS 11 hdd 3.00000 0.79999 2794G 2503G 290G 89.59 1.33 38 [@~]# ceph osd reweight 11 0.80001 reweighted osd.11 to 0.80001 (cccd) [@~]# ceph osd df ID CLASS WEIGHT REWEIGHT SIZE USE AVAIL %USE VAR PGS 11 hdd 3.00000 0.80000 2794G 2503G 290G 89.59 1.33 38 _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com <http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com> _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com <http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com> -- -- Paul Emmerich Looking for help with your Ceph cluster? Contact us at https://croit.io croit GmbH Freseniusstr. 31h 81247 München www.croit.io Tel: +49 89 1896585 90 _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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