We have had some bad issues when doing to much work in the reduce like described here, and reach some memory limit that killed the map/reduce.
“the size limit on the composite structure within the reduce() function is 64KB.”
General advice from manual :
“The reduce() function is designed to reduce and summarize the data emitted during the map() phase of the process. It should only be used to summarize the data, and not to transform the output information or concatenate the information into a single structure.”
By the way, there is some great solutions available for storing timeseries
graphite is one of them and could be considered : http://graphite.wikidot.com/faq#toc0
It stores data, but allo has a full set of APIs to render the data (png, json, etc …)
Xavier