On Sat, 23 Jan 2016, Xusangdi wrote:
> Hi Sage,
>
> Recently we encountered an interesting case when learning about CRUSH, please see below:
>
> root root {
> id -4 # do not change unnecessarily
> # weight 36.000
> alg straw2
> hash 0 # rjenkins1
> item host0 weight 3.000
> item host1 weight 3.000
> item host2 weight 30.000
> }
>
> CRUSHCHOOSE_LEAF bucket -4 x 3 outpos 0 numrep 3 tries 51 recurse_tries 1 local_retries 0 local_fallback_retries 0 parent_r 0 stable 0
> crush_bucket_choose -4 x=3 r=0
> item -3 type 1
> CHOOSE bucket -3 x 3 outpos 0 numrep 1 tries 1 recurse_tries 0 local_retries 0 local_fallback_retries 0 parent_r 0 stable 0
> crush_bucket_choose -3 x=3 r=0
> item 8 type 0
> CHOOSE got 8
> CHOOSE returns 1
> CHOOSE got -3
> crush_bucket_choose -4 x=3 r=1
> item -3 type 1
> reject 0 collide 1 ftotal 1 flocal 1
> crush_bucket_choose -4 x=3 r=2
> item -3 type 1
> reject 0 collide 1 ftotal 2 flocal 1
> crush_bucket_choose -4 x=3 r=3
> item -3 type 1
> reject 0 collide 1 ftotal 3 flocal 1
> crush_bucket_choose -4 x=3 r=4
> item -3 type 1
> reject 0 collide 1 ftotal 4 flocal 1
> crush_bucket_choose -4 x=3 r=5
> item -3 type 1
> reject 0 collide 1 ftotal 5 flocal 1
> crush_bucket_choose -4 x=3 r=6
> item -3 type 1
> reject 0 collide 1 ftotal 6 flocal 1
> crush_bucket_choose -4 x=3 r=7
> item -3 type 1
> reject 0 collide 1 ftotal 7 flocal 1
> crush_bucket_choose -4 x=3 r=8
> item -3 type 1
> reject 0 collide 1 ftotal 8 flocal 1
> crush_bucket_choose -4 x=3 r=9
> item -3 type 1
> reject 0 collide 1 ftotal 9 flocal 1
> crush_bucket_choose -4 x=3 r=10
> item -3 type 1
> reject 0 collide 1 ftotal 10 flocal 1
> crush_bucket_choose -4 x=3 r=11
> item -3 type 1
> reject 0 collide 1 ftotal 11 flocal 1
> crush_bucket_choose -4 x=3 r=12
> item -3 type 1
> reject 0 collide 1 ftotal 12 flocal 1
> crush_bucket_choose -4 x=3 r=13
> item -3 type 1
> reject 0 collide 1 ftotal 13 flocal 1
> crush_bucket_choose -4 x=3 r=14
> item -3 type 1
> reject 0 collide 1 ftotal 14 flocal 1
> crush_bucket_choose -4 x=3 r=15
> item -3 type 1
> reject 0 collide 1 ftotal 15 flocal 1
> crush_bucket_choose -4 x=3 r=16
> item -3 type 1
> reject 0 collide 1 ftotal 16 flocal 1
> crush_bucket_choose -4 x=3 r=17
> item -3 type 1
> reject 0 collide 1 ftotal 17 flocal 1
> crush_bucket_choose -4 x=3 r=18
> item -3 type 1
> reject 0 collide 1 ftotal 18 flocal 1
> crush_bucket_choose -4 x=3 r=19
> item -2 type 1
> CHOOSE bucket -2 x 3 outpos 1 numrep 2 tries 1 recurse_tries 0 local_retries 0 local_fallback_retries 0 parent_r 0 stable 0
> crush_bucket_choose -2 x=3 r=1
> item 4 type 0
> CHOOSE got 4
> CHOOSE returns 2
> CHOOSE got -2
> crush_bucket_choose -4 x=3 r=2
> item -3 type 1
> reject 0 collide 1 ftotal 1 flocal 1
> crush_bucket_choose -4 x=3 r=3
> item -3 type 1
> reject 0 collide 1 ftotal 2 flocal 1
> crush_bucket_choose -4 x=3 r=4
> item -3 type 1
> reject 0 collide 1 ftotal 3 flocal 1
> crush_bucket_choose -4 x=3 r=5
> item -3 type 1
> reject 0 collide 1 ftotal 4 flocal 1
> crush_bucket_choose -4 x=3 r=6
> item -3 type 1
> reject 0 collide 1 ftotal 5 flocal 1
> crush_bucket_choose -4 x=3 r=7
> item -3 type 1
> reject 0 collide 1 ftotal 6 flocal 1
> crush_bucket_choose -4 x=3 r=8
> item -3 type 1
> reject 0 collide 1 ftotal 7 flocal 1
> crush_bucket_choose -4 x=3 r=9
> item -3 type 1
> reject 0 collide 1 ftotal 8 flocal 1
> crush_bucket_choose -4 x=3 r=10
> item -3 type 1
> reject 0 collide 1 ftotal 9 flocal 1
> crush_bucket_choose -4 x=3 r=11
> item -3 type 1
> reject 0 collide 1 ftotal 10 flocal 1
> crush_bucket_choose -4 x=3 r=12
> item -3 type 1
> reject 0 collide 1 ftotal 11 flocal 1
> crush_bucket_choose -4 x=3 r=13
> item -3 type 1
> reject 0 collide 1 ftotal 12 flocal 1
> crush_bucket_choose -4 x=3 r=14
> item -3 type 1
> reject 0 collide 1 ftotal 13 flocal 1
> crush_bucket_choose -4 x=3 r=15
> item -3 type 1
> reject 0 collide 1 ftotal 14 flocal 1
> crush_bucket_choose -4 x=3 r=16
> item -3 type 1
> reject 0 collide 1 ftotal 15 flocal 1
> crush_bucket_choose -4 x=3 r=17
> item -3 type 1
> reject 0 collide 1 ftotal 16 flocal 1
> crush_bucket_choose -4 x=3 r=18
> item -3 type 1
> reject 0 collide 1 ftotal 17 flocal 1
> crush_bucket_choose -4 x=3 r=19
> item -2 type 1
> reject 0 collide 1 ftotal 18 flocal 1
> crush_bucket_choose -4 x=3 r=20
> item -2 type 1
> reject 0 collide 1 ftotal 19 flocal 1
> crush_bucket_choose -4 x=3 r=21
> item -3 type 1
> reject 0 collide 1 ftotal 20 flocal 1
> crush_bucket_choose -4 x=3 r=22
> item -3 type 1
> reject 0 collide 1 ftotal 21 flocal 1
> crush_bucket_choose -4 x=3 r=23
> item -1 type 1
> CHOOSE bucket -1 x 3 outpos 2 numrep 3 tries 1 recurse_tries 0 local_retries 0 local_fallback_retries 0 parent_r 0 stable 0
> crush_bucket_choose -1 x=3 r=2
> item 0 type 0
> CHOOSE got 0
> CHOOSE returns 3
> CHOOSE got -1
> CHOOSE returns 3
> rule 0 x 3 [8,4,0]
>
> It looks that when choosing the third replica, we repeat a lot of tries which have already tried for the second one.
> Is this intended or maybe an issue?
It's because one bucket is weighted so much more heavily than the others.
You're asking for something that's somewhat impossible: 3 replicas coming
from 3 items with different weights. Since we're sampling, if the weights
are too skewed you'll run out of tries (r values) and it'll give up with
only 2 results.
sage
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