Re: ceph osd reweight (doing -1 or actually -0.0001)

[Paul Emmerich <paul.emmerich@xxxxxxxx>]

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




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Paul Emmerich

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