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  1. Core Server
  2. SERVER-7850

Config server floods log with: CMD fsync: sync:1 lock:0

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    • Type: Icon: Bug Bug
    • Resolution: Duplicate
    • Priority: Icon: Major - P3 Major - P3
    • None
    • Affects Version/s: 2.2.1
    • Component/s: None
    • None
    • Linux

      We have a sharded cluster of 5 mongod's. They are started like this:

      dbpath = /mnt/mongo/rs0
shardsvr = true
maxConns = 20000
keyFile = keyfile

# Logging
logpath = logs/mongod-cn60.log

# Replica set
replSet = rs0
oplogSize = 1024

      These are connected to by 96 nodes running mongos.

      There are three config servers are run like:

      dbpath = /mnt/mongo/cs0
configsvr = true
keyFile = keyfile

# Logging
logpath = configserver-cn60.log

      We see the config server log file flooded with:

      Wed Dec  5 18:51:44 [conn583] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn584] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn813] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn756] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn289] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn828] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn561] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn244] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn823] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn168] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn651] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn1286] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn630] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn773] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn230] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn583] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn756] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn813] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn584] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn1320] CMD fsync: sync:1 lock:0
Wed Dec  5 18:51:44 [conn126] CMD fsync: sync:1 lock:0

      The config server has continuous 30% usage with no load on the cluster.

      It could be that each mongos is trying to balance, and this causes the config server to be permanently loaded. Disabling balancing reduces the load considerably to 5-10% CPU (cluster still not loaded). However we still see this fsync command every 30s or so from every connection.

      Is this expected? Can we enable balancing on only one mongos?

            Assignee:
            andre.defrere@mongodb.com Andre de Frere
            Reporter:
            jblackburn James Blackburn
            Votes:
            1 Vote for this issue
            Watchers:
            5 Start watching this issue

              Created:
              Updated:
              Resolved: