Zookeeper drops connections under moderate IO load

                 Key: ZOOKEEPER-885
                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-885
             Project: Zookeeper
          Issue Type: Bug
          Components: server
    Affects Versions: 3.2.2
         Environment: Debian (Lenny)
swap disabled
100Mb heap for zookeeper
            Reporter: Alexandre Hardy
            Priority: Critical

A zookeeper server under minimum load, with a number of clients watching 
exactly one node will fail to maintain the connection when the machine is 
subjected to moderate IO load.

In a specific test example we had three zookeeper servers running on dedicated 
machines with 45 clients connected, watching exactly one node. The clients 
would disconnect after moderate load was added to each of the zookeeper servers 
with the command:
dd if=/dev/urandom of=/dev/mapper/nimbula-test

The {{dd}} command transferred data at a rate of about 4Mb/s.

The same thing happens with
dd if=/dev/zero of=/dev/mapper/nimbula-test
transferring at a rate of about 20Mb/s. 

It seems strange that such a moderate load should cause instability in the 

Very few other processes were running, the machines were setup to test the 
connection instability we have experienced. Clients performed no other read or 
mutation operations.

Although the documents state that minimal competing IO load should present on 
the zookeeper server, it seems reasonable that moderate IO should not cause 
problems in this case.

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