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We using standard mongo setup: 2 replicas + 1 arbiter. Both replica servers use same AWS m1.medium with RAID10 EBS.

We experiencing constantly growing replication lag on secondary replica.

enter image description here

I tried to do full-resync, you can see it on graph, but it helped only for some hours. Our mongo usage is really low now, and frankly i can't understan why it can be.

iostat 1 for secondary:

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
          80.39    0.00    2.94    0.00   16.67    0.00

Device:            tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn
xvdap1            0.00         0.00         0.00          0          0
xvdb              0.00         0.00         0.00          0          0
xvdfp4           12.75         0.00       189.22          0        193
xvdfp3           12.75         0.00       189.22          0        193
xvdfp2            7.84         0.00        40.20          0         41
xvdfp1            7.84         0.00        40.20          0         41
md127            19.61         0.00       219.61          0        224

mongostat for secondary (why 100% locks? i guess its the problem):

insert  query update delete getmore command flushes mapped  vsize    res faults locked % idx miss %     qr|qw   ar|aw  netIn netOut  conn        set repl       time 
   *10     *0    *16     *0       0     2|4       0  30.9g  62.4g  1.65g      0      107          0       0|0     0|0   198b     1k    16 replset-01  SEC   06:55:37 
    *4     *0     *8     *0       0    12|0       0  30.9g  62.4g  1.65g      0     91.7          0       0|0     0|0   837b     5k    16 replset-01  SEC   06:55:38 
    *4     *0     *7     *0       0     3|0       0  30.9g  62.4g  1.64g      0      110          0       0|0     0|0   342b     1k    16 replset-01  SEC   06:55:39 
    *4     *0     *8     *0       0     1|0       0  30.9g  62.4g  1.64g      0     82.9          0       0|0     0|0    62b     1k    16 replset-01  SEC   06:55:40 
    *3     *0     *7     *0       0     5|0       0  30.9g  62.4g   1.6g      0     75.2          0       0|0     0|0   466b     2k    16 replset-01  SEC   06:55:41 
    *4     *0     *7     *0       0     1|0       0  30.9g  62.4g  1.64g      0      138          0       0|0     0|1    62b     1k    16 replset-01  SEC   06:55:42 
    *7     *0    *15     *0       0     3|0       0  30.9g  62.4g  1.64g      0     95.4          0       0|0     0|0   342b     1k    16 replset-01  SEC   06:55:43 
    *7     *0    *14     *0       0     1|0       0  30.9g  62.4g  1.64g      0       98          0       0|0     0|0    62b     1k    16 replset-01  SEC   06:55:44 
    *8     *0    *17     *0       0     3|0       0  30.9g  62.4g  1.64g      0     96.3          0       0|0     0|0   342b     1k    16 replset-01  SEC   06:55:45 
    *7     *0    *14     *0       0     3|0       0  30.9g  62.4g  1.64g      0     96.1          0       0|0     0|0   186b     2k    16 replset-01  SEC   06:55:46

mongostat for primary

insert  query update delete getmore command flushes mapped  vsize    res faults locked % idx miss %     qr|qw   ar|aw  netIn netOut  conn        set repl       time 
    12     30     20      0       0       3       0  30.9g  62.6g   641m      0      0.9          0       0|0     0|0   212k   619k    48 replset-01    M   06:56:41 
     5     17     10      0       0       2       0  30.9g  62.6g   641m      0      0.5          0       0|0     0|0   159k   429k    48 replset-01    M   06:56:42 
     9     22     16      0       0       3       0  30.9g  62.6g   642m      0      0.7          0       0|0     0|0   158k   276k    48 replset-01    M   06:56:43 
     6     18     12      0       0       2       0  30.9g  62.6g   640m      0      0.7          0       0|0     0|0    93k   231k    48 replset-01    M   06:56:44 
     6     12      8      0       0       3       0  30.9g  62.6g   640m      0      0.3          0       0|0     0|0    80k   125k    48 replset-01    M   06:56:45 
     8     21     14      0       0       9       0  30.9g  62.6g   641m      0      0.6          0       0|0     0|0   118k   419k    48 replset-01    M   06:56:46 
    10     34     20      0       0       6       0  30.9g  62.6g   640m      0      1.3          0       0|0     0|0   164k   527k    48 replset-01    M   06:56:47 
     6     21     13      0       0       2       0  30.9g  62.6g   641m      0      0.7          0       0|0     0|0   111k   477k    48 replset-01    M   06:56:48 
     8     21     15      0       0       2       0  30.9g  62.6g   641m      0      0.7          0       0|0     0|0   204k   336k    48 replset-01    M   06:56:49 
     4     12      8      0       0       8       0  30.9g  62.6g   641m      0      0.5          0       0|0     0|0   156k   530k    48 replset-01    M   06:56:50 

Mongo version: 2.0.6

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up vote 1 down vote accepted

Ok, we found the problem. We not the first, it was capped collections (logging). Actually i checked this version before this post, but miss point that indexes did't get sync automatically.

It means that you have to stop replica slave member, remove it from replica (just remove replicaSet from mongo.config), start and add index to capped collections: db.logs.ensureIndex({ _id: 1 }, { unique: true }), and return this instance to replica.

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Just to note, this is mentioned in the docs here: mongodb.org/display/DOCS/… –  Adam C Nov 3 '12 at 11:55
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