we have three data centres. Master is in one , and we have pairs of secondaries in the other two - these have priority = 0 and sometimes votes = 0. Only one of the secondaries in each data centre is read from. Today I issued a reindex on the secondary that was not being read from and replication to the one that was being read from halted. As soon as the reindex completed the other secondary started syncing again. Servers in other data centres were not affected and continued syncing. Here is a snippet of the mongostat:

localhost:3003 0 1239 301 0 96 7 0 140g 141g 12g 3 0 0|0 0|0 254k 4m
mongo03.lon.mongo:3003 *9 *0 *335 *0 11 3|0 0 140g 140g 12g 5.7 0 0|0 1|0 745b 240k 12
mongo04.lon.mongo:3003 *14 1677 *495 *0 0 3|0 0 140g 141g 11.7g 30 0 0|0 0|0 223k 6m 685
mongo09.wdc.mongo:3003 0 1197 297 0 93 7 0 140g 141g 12g 6.1 0 0|0 0|0 248k 4m 929
mongo10.wdc.mongo:3003 *6 2 *275 *0 11 7|0 0 142g 142g 12g 0 0 0|1 1|1 1k 138k 16
mongo74.ams01.mongo:3003 *0 *0 *0 *0 0 3|0 0 140g 140g 6.12g 0 0 0|0 1|0 275b 3k 13
mongo75.ams01.mongo:3003 *0 1640 *0 *0 0 4|0 0 140g 140g 7.81g 0 0 0|0 0|0 189k 5m 120

localhost:3003 0 1205 307 0 107 6 0 140g 141g 12g 4.6 0 0|0 0|0 253k 3m
mongo03.lon.mongo:3003 *9 *0 *337 *0 12 6|0 0 140g 140g 12g 8.5 0 0|0 1|0 1k 168k 12
mongo04.lon.mongo:3003 *8 1738 *318 *0 0 6|0 0 140g 141g 11.7g 7.1 0 1|0 1|0 200k 5m 685
mongo09.wdc.mongo:3003 0 1267 332 0 110 6 0 140g 141g 12g 1.5 0 0|0 0|0 269k 3m 929
mongo10.wdc.mongo:3003 *9 *0 *325 *0 11 7|0 0 142g 142g 12g 0 0 0|1 1|1 1k 166k 16
mongo74.ams01.mongo:3003 *0 *0 *0 *0 0 6|0 0 140g 140g 6.12g 0 0 0|0 1|0 730b 3k 13
mongo75.ams01.mongo:3003 *0 1775 *0 *0 0 6|0 0 140g 140g 7.81g 0 0 0|0 0|0 204k 5m 120

localhost:3003 0 1389 355 0 102 8 0 140g 141g 12g 0 0 0|0 0|0 290k 4m
mongo03.lon.mongo:3003 *9 *0 *338 *0 11 3|0 0 140g 140g 12g 14.2 0 0|0 1|0 792b 159k 12
mongo04.lon.mongo:3003 *10 1695 *341 *0 0 3|0 0 140g 141g 11.7g 11.2 0 0|0 0|0 195k 5m 685
mongo09.wdc.mongo:3003 0 1360 348 0 100 8 0 140g 141g 12g 3 0 0|0 1|0 284k 4m 929
mongo10.wdc.mongo:3003 *10 *0 *319 *0 10 6|0 0 142g 142g 12g 0 0 0|1 1|1 1k 147k 16
mongo74.ams01.mongo:3003 *0 *0 *0 *0 1 7|0 0 140g 140g 6.12g 0 0 0|0 1|0 591b 7k 14
mongo75.ams01.mongo:3003 *0 1621 *0 *0 0 3|0 0 140g 140g 7.81g 0 0 0|0 0|0 186k 5m 120

localhost:3003 0 1327 360 0 109 5 0 140g 141g 12g 3 0 1|0 1|0 285k 4m
mongo03.lon.mongo:3003 *14 *0 *375 *0 13 6|0 0 140g 140g 12g 11.4 0 0|0 1|0 1k 195k 12
mongo04.lon.mongo:3003 *14 1689 *391 *0 0 6|0 0 140g 141g 11.7g 13 0 0|0 0|0 195k 5m 685
mongo09.wdc.mongo:3003 0 1343 359 0 110 6 0 140g 141g 12g 3 0 0|0 0|0 286k 4m 929
mongo10.wdc.mongo:3003 *13 *0 *355 *0 11 7|0 0 142g 142g 12g 0 0 0|0 1|0 1k 179k 16
mongo74.ams01.mongo:3003 *0 *0 *0 *0 0 6|0 0 140g 140g 6.12g 0 0 0|0 1|0 730b 3k 14
mongo75.ams01.mongo:3003 *0 1621 *0 *0 0 3|0 0 140g 140g 7.81g 0 0 0|0 0|0 186k 5m 120

localhost:3003 0 1231 311 0 101 8 0 140g 141g 12g 3 0 0|0 0|0 255k 4m
mongo03.lon.mongo:3003 *12 *0 *318 *0 12 4|0 0 140g 140g 12g 11.4 0 0|0 1|0 990b 157k 12
mongo04.lon.mongo:3003 *12 1740 *312 *0 0 3|0 0 140g 141g 11.7g 14 0 8|0 8|0 199k 5m 685
mongo09.wdc.mongo:3003 0 1242 299 0 99 8 0 140g 141g 12g 0 0 0|0 1|0 252k 5m 930
mongo10.wdc.mongo:3003 *11 *0 *277 *0 11 5|0 0 142g 142g 12g 0 0 0|0 0|0 1k 197k 16
mongo74.ams01.mongo:3003 *5 *0 *62 *0 0 6|0 0 140g 140g 6.12g 11.4 0 1|0 1|0 730b 3k 14
mongo75.ams01.mongo:3003 *0 1595 *0 *0 0 6|0 0 140g 140g 7.81g 0 0 0|0 0|0 184k 5m 120

insert query update delete getmore command flushes mapped vsize res locked % idx miss % qr|qw ar|aw netIn netOut c
localhost:3003 0 1332 320 0 110 6 0 140g 141g 12g 6.1 0 0|0 1|0 270k 5m
mongo03.lon.mongo:3003 *8 *0 *316 *0 9 5|0 0 140g 140g 12g 19.9 0 1|0 1|0 1k 136k 12
mongo04.lon.mongo:3003 *8 1643 *281 *0 0 6|0 0 140g 141g 11.7g 10.1 0 0|0 0|0 190k 5m 685
mongo09.wdc.mongo:3003 0 1310 352 0 112 6 0 140g 141g 12g 7.6 0 0|0 1|0 280k 4m 930
mongo10.wdc.mongo:3003 *9 *0 3|330 *0 12 8|0 0 142g 142g 12g 15.3 0 0|0 0|0 1k 4m 16
mongo74.ams01.mongo:3003 *4 *0 *29 *0 1 3|0 0 140g 140g 6.13g 5.7 0 0|0 1|0 322b 49k 14
mongo75.ams01.mongo:3003 *9 1709 *91 *0 0 3|0 0 140g 140g 7.81g 4.2 0 0|0 0|0 196k 6m 120

localhost:3003 0 1185 350 0 98 8 0 140g 141g 12g 3 0 0|0 1|0 264k 4m
mongo03.lon.mongo:3003 *9 *0 *370 *0 7 5|0 0 140g 140g 12g 44.2 0 1|0 1|0 755b 196k 12
mongo04.lon.mongo:3003 *9 1541 *404 *0 0 3|0 0 140g 141g 11.7g 12.8 0 0|0 0|0 177k 4m 685
mongo09.wdc.mongo:3003 0 1142 322 0 98 7 0 140g 141g 12g 3 0 0|0 1|0 250k 4m 930
mongo10.wdc.mongo:3003 *9 *0 *343 *0 5 5|0 0 142g 142g 12g 0 0 0|0 0|0 725b 165k 16
mongo74.ams01.mongo:3003 *52 *0 *1493 *0 0 6|0 0 140g 140g 6.14g 78.5 0 1|0 1|0 730b 3k 14
mongo75.ams01.mongo:3003 *0 1600 *0 *0 0 7|0 0 140g 140g 7.81g 0 0 0|0 0|0 184k 6m 120

localhost:3003 0 1207 328 0 102 5 0 140g 141g 12g 3 0 0|0 0|0 259k 4m
mongo03.lon.mongo:3003 *8 *0 *358 *0 12 4|0 0 140g 140g 12g 15.7 0 0|0 1|0 1k 179k 12
mongo04.lon.mongo:3003 *8 1671 *365 *0 0 6|0 0 140g 141g 11.7g 12.8 0 0|0 0|0 192k 5m 685
mongo09.wdc.mongo:3003 0 1301 336 0 105 6 0 140g 141g 12g 3 0 0|0 0|0 273k 5m 930
mongo10.wdc.mongo:3003 *6 *0 *301 *0 10 7|0 0 142g 142g 12g 0 0 0|0 0|0 1k 152k 16
mongo74.ams01.mongo:3003 *82 *0 *2220 *0 0 3|0 0 140g 140g 6.15g 100 0 1|0 1|0 275b 3k 14
mongo75.ams01.mongo:3003 *0 1603 *0 *0 0 2|0 0 140g 140g 7.81g 0 0 0|0 0|0 184k 6m 120

localhost:3003 0 1356 323 0 106 8 0 140g 141g 12g 4.6 0 0|0 0|0 275k 5m
mongo03.lon.mongo:3003 *10 *0 *343 *0 12 5|0 0 140g 140g 12g 11.4 0 0|0 1|0 1k 169k 12
mongo04.lon.mongo:3003 *10 1744 *343 *0 0 4|0 0 140g 141g 11.7g 8.5 0 0|0 0|0 200k 5m 684
mongo09.wdc.mongo:3003 0 1325 327 0 110 7 0 140g 141g 12g 6.1 0 0|0 0|0 273k 5m 930
mongo10.wdc.mongo:3003 *7 *0 *326 *0 11 5|0 0 142g 142g 12g 0 0 0|0 0|0 1k 162k 16
mongo74.ams01.mongo:3003 *99 *0 *2330 *0 0 6|0 0 140g 140g 6.16g 98.5 0 1|0 1|0 730b 3k 14
mongo75.ams01.mongo:3003 *0 1600 *0 *0 0 7|0 0 140g 140g 7.81g 0 0 0|0 0|0 184k 5m 120

localhost:3003 0 1265 333 0 116 6 0 140g 141g 12g 7.6 0 0|0 0|0 268k 5m
mongo03.lon.mongo:3003 *10 *0 *343 *0 12 5|0 0 140g 140g 12g 11.4 0 0|0 1|0 1k 169k 12
mongo04.lon.mongo:3003 *10 1744 *343 *0 0 4|0 0 140g 141g 11.7g 8.5 0 0|0 0|0 200k 5m 684
mongo09.wdc.mongo:3003 0 1205 327 0 112 6 0 140g 141g 12g 7.6 0 0|0 0|0 258k 4m 930
mongo10.wdc.mongo:3003 *7 *0 *337 *0 11 8|0 0 142g 142g 12g 0 0 0|0 0|0 1k 169k 16
mongo74.ams01.mongo:3003 *92 *0 *2444 *0 0 3|0 0 140g 140g 6.18g 98.5 0 2|0 1|0 275b 3k 14
mongo75.ams01.mongo:3003 *0 1611 *0 *0 0 3|0 0 140g 140g 7.81g 0 0 0|0 0|0 185k 5m 120

insert query update delete getmore command flushes mapped vsize res locked % idx miss % qr|qw ar|aw netIn netOut c
localhost:3003 0 1188 356 0 109 6 0 140g 141g 12g 6.1 0 0|0 1|0 266k 4m
mongo03.lon.mongo:3003 *3 *0 *365 *0 12 4|0 0 140g 140g 12g 11.4 0 0|0 1|0 990b 177k 12
mongo04.lon.mongo:3003 *3 1669 *345 *0 0 5|0 0 140g 141g 11.7g 15.4 0 3|0 3|0 192k 5m 684
mongo09.wdc.mongo:3003 0 1217 360 0 116 7 0 140g 141g 12g 9.2 0 0|0 1|1 272k 4m 930
mongo10.wdc.mongo:3003 *3 *0 *343 *0 12 5|0 0 142g 142g 12g 1.5 0 0|0 0|0 1k 4m 16
mongo74.ams01.mongo:3003 *92 *0 *2444 *0 0 3|0 0 140g 140g 6.18g 98.5 0 2|0 1|0 275b 3k 14
mongo75.ams01.mongo:3003 *41 1720 *1102 *0 0 6|0 0 140g 140g 7.81g 42.2 0 4|1 5|1 197k 6m 120

localhost:3003 0 1286 337 0 117 19 0 140g 141g 12g 12.3 0 0|0 0|0 273k 9m
mongo03.lon.mongo:3003 *8 *0 *350 *0 11 5|0 0 140g 140g 12g 11.4 0 0|0 1|0 1k 171k 12
mongo04.lon.mongo:3003 *8 1712 1|370 *0 0 6|0 0 140g 141g 11.7g 14.4 0 0|0 0|0 197k 5m 684
mongo09.wdc.mongo:3003 0 1323 271 0 90 18 0 140g 141g 12g 6.1 0 0|0 1|0 253k 9m 930
mongo10.wdc.mongo:3003 *9 *0 *312 *0 10 9|0 0 142g 142g 12g 0 0 0|0 0|0 1k 142k 16
mongo74.ams01.mongo:3003 *62 *0 *1835 *0 0 14|0 0 140g 140g 6.19g 72.8 0 0|0 0|0 1k 4m 14
mongo75.ams01.mongo:3003 *74 1616 *2200 *0 0 3|0 0 140g 140g 7.81g 87.3 0 5|0 5|0 186k 5m 122

localhost:3003 0 1119 52 0 18 6 0 140g 141g 12g 0 0 0|0 20|0 146k 4m
mongo03.lon.mongo:3003 *5 *0 *248 *0 9 4|0 0 140g 140g 12g 12.8 0 0|0 1|0 849b 123k 12
mongo04.lon.mongo:3003 *5 1698 *248 *0 0 4|0 0 140g 141g 11.7g 11.4 0 0|0 1|0 195k 4m 683
mongo09.wdc.mongo:3003 0 922 33 0 1 7 0 140g 141g 12g 0 0 64|30 106|30 94k 2m 930
mongo10.wdc.mongo:3003 *2 *0 *108 *0 6 4|0 0 142g 142g 12g 0 0 0|0 1|0 572b 71k 16
mongo74.ams01.mongo:3003 *80 *0 *2626 *0 0 5|0 0 140g 140g 6.2g 100 0 0|0 0|0 407b 5k 14
mongo75.ams01.mongo:3003 *100 1654 *2318 *0 0 14|0 0 140g 140g 7.81g 92.9 0 7|1 8|1 191k 5m 122