[ceph-users] Difference between "ceph osd reweight" and "ceph osd crush reweight"

Gregory Farnum greg at inktank.com
Thu Jun 26 10:05:00 PDT 2014


On Thu, Jun 26, 2014 at 7:03 AM, Micha Krause <micha at krausam.de> wrote:
> Hi,
>
> could someone explain to me what the difference is between
>
> ceph osd reweight
>
> and
>
> ceph osd crush reweight

"ceph osd crush reweight" sets the CRUSH weight of the OSD. This
weight is an arbitrary value (generally the size of the disk in TB or
something) and controls how much data the system tries to allocate to
the OSD.

"ceph osd reweight" sets an override weight on the OSD. This value is
in the range 0 to 1, and forces CRUSH to re-place (1-weight) of the
data that would otherwise live on this drive. It does *not* change the
weights assigned to the buckets above the OSD, and is a corrective
measure in case the normal CRUSH distribution isn't working out quite
right. (For instance, if one of your OSDs is at 90% and the others are
at 50%, you could reduce this weight to try and compensate for it.)

It looks like our docs aren't very clear on the difference, when it
even mentions them...and admittedly it's a pretty subtle issue!
-Greg
Software Engineer #42 @ http://inktank.com | http://ceph.com


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