i think there is a wiki page on this, but for the short answer:

the number of znodes impact two things: memory footprint and recovery time. there is a base overhead to znodes to store its path, pointers to the data, pointers to the acl, etc. i believe that is around 100 bytes. you cant just divide your memory by 100+1K (for data) though, because the GC needs to be able to run and collect things and maintain a free space. if you use 3/4 of your available memory, that would mean with 4G you can store about three million znodes. when there is a crash and you recover, servers may need to read this data back off the disk or over the network. that means it will take about a minute to read 3G from the disk and perhaps a bit more to read it over the network, so you will need to adjust your initLimit accordingly.

of course this is all back-of-the-envelope. i would suggest doing some quick benchmarks to test and make sure your results are in line with expectation.

ben

On 07/15/2010 02:56 AM, Maarten Koopmans wrote:
Hi,

I am mapping a filesystem to ZooKeeper, and use it for locking and mapping a 
filesystem namespace to a flat data object space (like S3). So assuming proper 
nesting and small ZooKeeper nodes (<  1KB), how many nodes could a cluster with 
a few GBs of memory per instance realistically hold totally?

Thanks, Maarten

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