On 02/13/2017 10:15 PM, Sage Weil wrote: > On Mon, 13 Feb 2017, Loic Dachary wrote: >> 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. > > Oh, right, now I remember. The movement for the second replica is > unavoidable (as far as I can see). For the second replica, sometimes we > end up picking a dup (the same thing we got for the first > replica) and trying again; any change in the behavior of the first choice > may mean that we have more or less "second tries." Although any given try > will behave as we like (only moving to or from the reweighted item), > adding new tries will pick uniformly. In your example below, I think all > of the second replicas that moved to osds 0-9 were objects that originally > picked a dup for the second try and, once 10 was added, did not--because > the first replica was now on the new osd 10. So this is another manifestation of the multipick anomaly ? > sage > >> 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 >>>>>>>> More majordomo info at http://vger.kernel.org/majordomo-info.html >>>>>>>> >>>>>>> >>>>>>> -- >>>>>>> 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 >> -- 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
