I get the expected behavior for replica 1 (which is what CRUSH.straw2_reweight does). The movement between buckets observered below is for replica 2.
00 01 02 03 04 05 06 07 08 09 10
00: 0 0 0 0 0 0 0 0 0 0 927
01: 0 0 0 0 0 0 0 0 0 0 904
02: 0 0 0 0 0 0 0 0 0 0 928
03: 0 0 0 0 0 0 0 0 0 0 886
04: 0 0 0 0 0 0 0 0 0 0 927
05: 0 0 0 0 0 0 0 0 0 0 927
06: 0 0 0 0 0 0 0 0 0 0 930
07: 0 0 0 0 0 0 0 0 0 0 842
08: 0 0 0 0 0 0 0 0 0 0 943
09: 0 0 0 0 0 0 0 0 0 0 904
10: 0 0 0 0 0 0 0 0 0 0 0
before: 10149 10066 9893 9955 10030 10025 9895 10013 10008 9966 0
after: 9222 9162 8965 9069 9103 9098 8965 9171 9065 9062 9118
On 02/13/2017 09:18 PM, Loic Dachary wrote:
>
>
> On 02/13/2017 08:16 PM, Sage Weil wrote:
>> On Mon, 13 Feb 2017, Loic Dachary wrote:
>>> Hi Sage,
>>>
>>> I wrote a little program to show where objects are moving when a new disk is added (disk 10 below) and it looks like this:
>>>
>>> 00 01 02 03 04 05 06 07 08 09 10
>>> 00: 0 14 17 14 19 23 13 22 21 20 1800
>>> 01: 12 0 11 13 19 19 15 10 16 17 1841
>>> 02: 17 27 0 17 15 15 13 19 18 11 1813
>>> 03: 14 17 15 0 23 11 20 15 23 17 1792
>>> 04: 14 18 16 25 0 27 13 8 15 16 1771
>>> 05: 19 16 22 25 13 0 9 19 21 21 1813
>>> 06: 18 15 21 17 10 18 0 10 18 11 1873
>>> 07: 13 17 22 13 16 17 14 0 25 12 1719
>>> 08: 23 20 16 17 19 18 11 12 0 18 1830
>>> 09: 14 20 15 17 12 16 17 11 13 0 1828
>>> 10: 0 0 0 0 0 0 0 0 0 0 0
>>>
>>> before: 20164 19990 19863 19959 19977 20004 19926 20133 20041 19943 0
>>> after: 18345 18181 18053 18170 18200 18190 18040 18391 18227 18123 18080
>>>
>>>
>>> Each line shows how many objects moved from a given disk to the others
>>> after disk 10 was added. Most objects go to the new disk and around 1%
>>> go to each other disks. The before and after lines show how many objects
>>> are mapped to each disk. They all have the same weight and it's using
>>> replica 2 and straw2. Does that look right ?
>>
>> Hmm, that doesn't look right. This is what the CRUSH.straw2_reweight unit
>> test is there to validate: that data on moves to or from the device whose
>> weight changed.
>
> In the above, the bucket size changes: it has a new item. And the bucket size plays a role in bucket_straw2_choose because it loops on all items. In CRUSH.straw2_reweight only the weights change. I'm not entirely sure how that would explain the results I get though...
>
>> It also follows from the straw2 algorithm itself: each possible choice
>> gets a 'straw' length derived only from that item's weight (and other
>> fixed factors, like the item id and the bucket id), and we select the max
>> across all items. Two devices whose weights didn't change will have the
>> same straw lengths, and the max between them will not change. It's only
>> possible that the changed item's straw length changed and wasn't max and
>> now is (got longer) or was max and now isn't (got shorter).
>
> That's a crystal clear explanation, cool :-)
>
> Cheers
>
>> sage
>>
>>
>>>
>>> Cheers
>>>
>>> On 02/13/2017 03:21 PM, Sage Weil wrote:
>>>> On Mon, 13 Feb 2017, Loic Dachary wrote:
>>>>> Hi,
>>>>>
>>>>> Dan van der Ster reached out to colleagues and friends and Pedro
>>>>> López-Adeva Fernández-Layos came up with a well written analysis of the
>>>>> problem and a tentative solution which he described at :
>>>>> https://github.com/plafl/notebooks/blob/master/replication.ipynb
>>>>>
>>>>> Unless I'm reading the document incorrectly (very possible ;) it also
>>>>> means that the probability of each disk needs to take in account the
>>>>> weight of all disks. Which means that whenever a disk is added / removed
>>>>> or its weight is changed, this has an impact on the probability of all
>>>>> disks in the cluster and objects are likely to move everywhere. Am I
>>>>> mistaken ?
>>>>
>>>> Maybe (I haven't looked closely at the above yet). But for comparison, in
>>>> the normal straw2 case, adding or removing a disk also changes the
>>>> probabilities for everything else (e.g., removing one out of 10 identical
>>>> disks changes the probability from 1/10 to 1/9). The key property that
>>>> straw2 *is* able to handle is that as long as the relative probabilities
>>>> between two unmodified disks does not change, then straw2 will avoid
>>>> moving any objects between them (i.e., all data movement is to or from
>>>> the disk that is reweighted).
>>>>
>>>> sage
>>>>
>>>>
>>>>>
>>>>> Cheers
>>>>>
>>>>> On 01/26/2017 04:05 AM, Sage Weil wrote:
>>>>>> This is a longstanding bug,
>>>>>>
>>>>>> http://tracker.ceph.com/issues/15653
>>>>>>
>>>>>> that causes low-weighted devices to get more data than they should. Loic's
>>>>>> recent activity resurrected discussion on the original PR
>>>>>>
>>>>>> https://github.com/ceph/ceph/pull/10218
>>>>>>
>>>>>> but since it's closed and almost nobody will see it I'm moving the
>>>>>> discussion here.
>>>>>>
>>>>>> The main news is that I have a simple adjustment for the weights that
>>>>>> works (almost perfectly) for the 2nd round of placements. The solution is
>>>>>> pretty simple, although as with most probabilities it tends to make my
>>>>>> brain hurt.
>>>>>>
>>>>>> The idea is that, on the second round, the original weight for the small
>>>>>> OSD (call it P(pick small)) isn't what we should use. Instead, we want
>>>>>> P(pick small | first pick not small). Since P(a|b) (the probability of a
>>>>>> given b) is P(a && b) / P(b),
>>>>>>
>>>>>> P(pick small | first pick not small)
>>>>>> = P(pick small && first pick not small) / P(first pick not small)
>>>>>>
>>>>>> The last term is easy to calculate,
>>>>>>
>>>>>> P(first pick not small) = (total_weight - small_weight) / total_weight
>>>>>>
>>>>>> and the && term is the distribution we're trying to produce. For exmaple,
>>>>>> if small has 1/10 the weight, then we should see 1/10th of the PGs have
>>>>>> their second replica be the small OSD. So
>>>>>>
>>>>>> P(pick small && first pick not small) = small_weight / total_weight
>>>>>>
>>>>>> Putting those together,
>>>>>>
>>>>>> P(pick small | first pick not small)
>>>>>> = P(pick small && first pick not small) / P(first pick not small)
>>>>>> = (small_weight / total_weight) / ((total_weight - small_weight) / total_weight)
>>>>>> = small_weight / (total_weight - small_weight)
>>>>>>
>>>>>> This is, on the second round, we should adjust the weights by the above so
>>>>>> that we get the right distribution of second choices. It turns out it
>>>>>> works to adjust *all* weights like this to get hte conditional probability
>>>>>> that they weren't already chosen.
>>>>>>
>>>>>> I have a branch that hacks this into straw2 and it appears to work
>>>>>> properly for num_rep = 2. With a test bucket of [99 99 99 99 4], and the
>>>>>> current code, you get
>>>>>>
>>>>>> $ bin/crushtool -c cm.txt --test --show-utilization --min-x 0 --max-x 40000000 --num-rep 2
>>>>>> rule 0 (data), x = 0..40000000, numrep = 2..2
>>>>>> rule 0 (data) num_rep 2 result size == 2: 40000001/40000001
>>>>>> device 0: 19765965 [9899364,9866601]
>>>>>> device 1: 19768033 [9899444,9868589]
>>>>>> device 2: 19769938 [9901770,9868168]
>>>>>> device 3: 19766918 [9898851,9868067]
>>>>>> device 6: 929148 [400572,528576]
>>>>>>
>>>>>> which is very close for the first replica (primary), but way off for the
>>>>>> second. With my hacky change,
>>>>>>
>>>>>> rule 0 (data), x = 0..40000000, numrep = 2..2
>>>>>> rule 0 (data) num_rep 2 result size == 2: 40000001/40000001
>>>>>> device 0: 19797315 [9899364,9897951]
>>>>>> device 1: 19799199 [9899444,9899755]
>>>>>> device 2: 19801016 [9901770,9899246]
>>>>>> device 3: 19797906 [9898851,9899055]
>>>>>> device 6: 804566 [400572,403994]
>>>>>>
>>>>>> which is quite close, but still skewing slightly high (by a big less than
>>>>>> 1%).
>>>>>>
>>>>>> Next steps:
>>>>>>
>>>>>> 1- generalize this for >2 replicas
>>>>>> 2- figure out why it skews high
>>>>>> 3- make this work for multi-level hierarchical descent
>>>>>>
>>>>>> sage
>>>>>>
>>>>>> --
>>>>>> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in
>>>>>> the body of a message to majordomo@xxxxxxxxxxxxxxx
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>>>>>>
>>>>>
>>>>> --
>>>>> Loïc Dachary, Artisan Logiciel Libre
>>>>> --
>>>>> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in
>>>>> the body of a message to majordomo@xxxxxxxxxxxxxxx
>>>>> More majordomo info at http://vger.kernel.org/majordomo-info.html
>>>>>
>>>
>>> --
>>> Loïc Dachary, Artisan Logiciel Libre
>
--
Loïc Dachary, Artisan Logiciel Libre
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