Alexandre Hardy commented on ZOOKEEPER-885:

Hi Flavio,

I've  set up some ec2 instances to reproduce the problem. I think the
problem is related to relative disk performance and load.

I have had to use a more aggressive disk benchmark utility to get the
problem to occur, and I realise that this is contrary to the zookeeper
requirements. However, I think we would like to know why a ping would
be affected when no disk access is expected.

Can we discuss access to these instances vie e-mail?

Kind regards

> 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, 3.3.1
>         Environment: Debian (Lenny)
> 1Gb RAM
> swap disabled
> 100Mb heap for zookeeper
>            Reporter: Alexandre Hardy
>            Priority: Critical
>         Attachments: benchmark.csv, tracezklogs.tar.gz, 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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