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From Patrick Hunt <ph...@apache.org>
Subject Re: size of data / number of znodes
Date Tue, 15 Dec 2009 17:36:13 GMT
See this recent benchmark I did: http://bit.ly/4ekN8G

In this case I have 20 clients doing 10k zodes each (200k znodes of size 
100 bytes each with 1million watches). However I have tested similar 
setup with 400 clients (so 4 million znodes and 20million watches).

As Ben mentioned there are no limits other than memory (obv cpu, disk 
size, network performance, etc... are also issues, but memory and memory 
management in particular are the most important).

I found the following critical in my testing:

1) use a recent JVM. Some of the older JVMs would crash (jvm fault, not 
our code) with these loads. After upgrading to 1.6.0_17 I no longer saw this

2) Provide sufficient memory in the JVM heap. Tune your GC -- in 
particular you need to turn on incremental/CMS GC in the JVM. Turn on GC 
logging so that if you do see issues you can review the GC logs for 
pauses. Keep in mind that CMS tends to fragment the heap. G1 will help 
this, but it's not ready in 1.6, hopefully it will be more stable in 1.7.

3) dedicated transactional log device (separate disk) if performance is 
critical. ie guaranteed low latency times.

See the troubleshooting page for some common issues: http://bit.ly/5WwS44


Benjamin Reed wrote:
> there aren't any limits on the number of znodes, it's just limited by 
> your memory. there are two things (probably more :) to keep in mind:
> 1) the 1M limit also applies to the children list. you can't grow the 
> list of children to more than 1M (the sum of the names of all of the 
> children) otherwise you cannot to a getChildren(). so, yes, you need to 
> do some bucketing to keep the number of children to something 
> reasonable. assuming your names will be less than 100 bytes, you 
> probably want to limit the number of children to 10,000.
> 2) since there are times that you need to do a state transfer between 
> servers (dump all the state from one to the other to bring it online) it 
> may take a while depending on your network speed. you may need to bump 
> up the default initLimit, so make sure you do some benchmarking on your 
> platform to test your configuration parameters.
> ben
> Michael Bauland wrote:
>> Hello,
>> I'm new to the Zookeeper project and wondering whether our use case is a
>> good one for Zookeeper. I read the documentation, but couldn't find an
>> answer. At some point it says that
>>> A common property of the various forms of coordination data is that 
>>> they are relatively small: measured in kilobytes. The ZooKeeper 
>>> client and the server implementations have sanity checks to ensure 
>>> that znodes have less than 1M of data
>> I couldn't find any limits on the number of znodes used, only that each
>> znode should only contain little data. We were planning to use a million
>> znodes (each containing a few hundred bytes of data). Would this use
>> case be acceptable for Zookeeper? And if so, does it matter if we have a
>> flat hierarchy (i.e, all nodes have the root node as their direct
>> ancestor) or should we introduce some (artificial) hierarchy levels to
>> have a more tree-like structure?
>> Thanks in advance for your answer.
>> Cheers,
>> Michael

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