On Tue, Sep 1, 2015 at 3:58 PM, huang jun <hjwsm1989@xxxxxxxxx> wrote: > hi,all > > Recently, i did some experiments on OSD data distribution, > we set up a cluster with 72 OSDs,all 2TB sata disk, > and ceph version is v0.94.3 and linux kernel version is 3.18, > and set "ceph osd crush tunables optimal". > There are 3 pools: > pool 0 'rbd' replicated size 2 min_size 1 crush_ruleset 0 object_hash > rjenkins pg_num 512 pgp_num 512 last_change 302 stripe_width 0 > pool 1 'data' replicated size 2 min_size 1 crush_ruleset 0 object_hash > rjenkins pg_num 4096 pgp_num 4096 last_change 832 > crash_replay_interval 45 stripe_width 0 > pool 2 'metadata' replicated size 3 min_size 1 crush_ruleset 0 > object_hash rjenkins pg_num 512 pgp_num 512 last_change 302 > stripe_width 0 > > the osd pg num of each osd: > pool : 0 1 2 | SUM > ---------------------------------------- > osd.0 13 105 18 | 136 > osd.1 17 110 26 | 153 > osd.2 15 114 20 | 149 > osd.3 11 101 17 | 129 > osd.4 8 106 17 | 131 > osd.5 12 102 19 | 133 > osd.6 19 114 29 | 162 > osd.7 16 115 21 | 152 > osd.8 15 117 25 | 157 > osd.9 13 117 23 | 153 > osd.10 13 133 16 | 162 > osd.11 14 105 21 | 140 > osd.12 11 94 16 | 121 > osd.13 12 110 21 | 143 > osd.14 20 119 26 | 165 > osd.15 12 125 19 | 156 > osd.16 15 126 22 | 163 > osd.17 13 109 19 | 141 > osd.18 8 119 19 | 146 > osd.19 14 114 19 | 147 > osd.20 17 113 29 | 159 > osd.21 17 111 27 | 155 > osd.22 13 121 20 | 154 > osd.23 14 95 23 | 132 > osd.24 17 110 26 | 153 > osd.25 13 133 15 | 161 > osd.26 17 124 24 | 165 > osd.27 16 119 20 | 155 > osd.28 19 134 30 | 183 > osd.29 13 121 20 | 154 > osd.30 11 97 20 | 128 > osd.31 12 109 18 | 139 > osd.32 10 112 15 | 137 > osd.33 18 114 28 | 160 > osd.34 19 112 29 | 160 > osd.35 16 121 32 | 169 > osd.36 13 111 18 | 142 > osd.37 15 107 22 | 144 > osd.38 21 129 24 | 174 > osd.39 9 121 17 | 147 > osd.40 11 102 18 | 131 > osd.41 14 101 19 | 134 > osd.42 16 119 25 | 160 > osd.43 12 118 13 | 143 > osd.44 17 114 25 | 156 > osd.45 11 114 15 | 140 > osd.46 12 107 16 | 135 > osd.47 15 111 23 | 149 > osd.48 14 115 20 | 149 > osd.49 9 94 13 | 116 > osd.50 14 117 18 | 149 > osd.51 13 112 19 | 144 > osd.52 11 126 22 | 159 > osd.53 12 122 18 | 152 > osd.54 13 121 20 | 154 > osd.55 17 114 25 | 156 > osd.56 11 118 18 | 147 > osd.57 22 137 25 | 184 > osd.58 15 105 22 | 142 > osd.59 13 120 18 | 151 > osd.60 12 110 19 | 141 > osd.61 21 114 28 | 163 > osd.62 12 97 18 | 127 > osd.63 19 109 31 | 159 > osd.64 10 132 21 | 163 > osd.65 19 137 21 | 177 > osd.66 22 107 32 | 161 > osd.67 12 107 20 | 139 > osd.68 14 100 22 | 136 > osd.69 16 110 24 | 150 > osd.70 9 101 14 | 124 > osd.71 15 112 24 | 151 > > ---------------------------------------- > SUM : 1024 8192 1536 | > > We can found that, for poolid=1(data pool), > osd.57 and osd.65 both have 137 PGs but osd.12 and osd.49 only have 94 PGs, > which maybe cause data distribution imbanlance, and reduces the space > utilization of the cluster. > > Use "crushtool -i crush.raw --test --show-mappings --rule 0 --num-rep > 2 --min-x 1 --max-x %s" > we tested different pool pg_num: > > Total PG num PG num stats > ------------ ------------------- > 4096 avg: 113.777778 (avg stands for avg PG num of every osd) > total: 8192 (total stands for total PG num, include replica PG) > max: 139 +0.221680 (max stands for max PG num on OSD, +0.221680 stands > for percent above avage PG num ) > min: 113 -0.226562 (min stands for min PG num on OSD, -0.226562 stands > for ratio below avage PG num ) > > 8192 avg: 227.555556 > total: 16384 > max: 267 0.173340 > min: 226 -0.129883 > > 16384 avg: 455.111111 > total: 32768 > max: 502 0.103027 > min: 455 -0.127686 > > 32768 avg: 910.222222 > total: 65536 > max: 966 0.061279 > min: 910 -0.076050 > > With bigger pg_num, the gap between the maximum and the minimum decreased. > But it's unreasonable to set such large pg_num, which will increase > OSD and MON load. > > Is there any way to get a more balanced PG distribution of the cluster? > We tried "ceph osd reweight-by-pg 110 data" many times, but that can > not resolve the problem. The numbers you're seeing here look broadly typical to me. We've explored a few ideas but have not found anything very satisfactory for providing more even distribution. So far it's just the cost of doing business with a pseudorandom placement algorithm. (And it's common to all storage systems using this mechanism, as far as I can tell.) > > Another problem is that if we can ensure the PG is distributed > balanced, can we ensure the data > distribution is balanced like PG? We haven't found this to be an issue the same way PG distribution is. Due to how objects are placed within PGs, your PGs will all tend to be of size X or of size 2*X, and the numbers of both will be large. (You can keep it to only size X by setting pgnum to be a power of 2, but I don't think it's worth worrying about much.) -Greg > > Btw, we will write data to this cluster until one or more osd get > full, we set full ratio to 0.98, > and we expect the cluster can use 0.9 total capacity. > > Any tips are welcome. > > -- > thanks > huangjun > _______________________________________________ > ceph-users mailing list > ceph-users@xxxxxxxxxxxxxx > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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