Per section 3.4.4 The default bucket type straw computes the hash of (PG
number, replica number, bucket id) for all buckets using the Jenkins
integer hashing function, then multiply this by bucket weight (for OSD
disks the weight of 1 is for 1 TB, for higher level it is the sum of
contained weights). The selection function chooses the bucket/disk with
the max value:
c(r,x) = maxi ( f (wi)hash(x, r, i))
So if you add a OSD disk, there is a new disk id that enters this
competition and will get PG from other OSDs proportional to its weight,
which is a desirable effect, but a side effect is that the weight
hierarchy has slightly changed so now some older buckets may win PGs
from other older buckets according to the hash function.
So straw does have overhead when adding (rather than replacing), it does
not do minimal PG re-assignments. But it terms of overall efficiency of
adding/removing of buckets at end and in middle of hierarchy it is the
best overall over other algorithms as seen on chart 5 and table 2.
On 2017-09-22 08:36, Will Zhao wrote:
> Hi Sage and all :
> I am tring to understand cursh more deeply. I have tried to read
> the code and paper, and search the mail list archives , but I still
> have some questions and can't understand it well.
> If I have 100 osds, and when I add a osd , the osdmap changes,
> and how the pg is recaulated to make sure the data movement is
> minimal. I tried to use crushtool --show-mappings --num-rep 3 --test
> -i map , through changing the map for 100osds and 101 osds , to look
> the result , it looks like the pgmap changed a lot . Shouldn't the
> remap only happen to some of the pgs ? Or crush from adding a pg is
> different from a new osdmap ? I konw I must understand something
> wrong. I appreciate if you can explain more about the logic of adding
> a osd . Or is there more doc that I can read ? Thank you very much
> !!! : )
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