Alexandre Hardy updated ZOOKEEPER-885:

    Attachment: tracezklogs.tar.gz

I accidentally missed the configuration for one of the nodes (switching from 
3.2.2 to 3.3.1). Thanks for spotting that.

Here are updated log files. I have updated the test program to terminate as 
soon as any single connection is dropped (Since my goal is to have zero 
connection failures for this simple test).

The client and server clocks are a bit out of sync (about 4 minutes, with the 
servers logging in UTC and client logging in UTC+2), but the test has been set 
up so that only one "instability event" is recorded.

> 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)
> 1Gb RAM
> swap disabled
> 100Mb heap for zookeeper
>            Reporter: Alexandre Hardy
>            Priority: Critical
>         Attachments: tracezklogs.tar.gz, WatcherTest.java, zklogs.tar.gz
> 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:
> {noformat}
> dd if=/dev/urandom of=/dev/mapper/nimbula-test
> {noformat}
> The {{dd}} command transferred data at a rate of about 4Mb/s.
> The same thing happens with
> {noformat}
> dd if=/dev/zero of=/dev/mapper/nimbula-test
> {noformat}
> It seems strange that such a moderate load should cause instability in the 
> connection.
> 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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