rrd tool alternative for high volume

I am interested in knowing if there is any alternative to rrdtool for logging time series data. I am looking at something that can scale for a large number of devices to monitor.

From what I read on this subject, rrdtool is I/O bound when you hit it with large amounts of data. Since I envision this to scale to a very large number of devices to monitor, I am curious if there's any alternative that would not choke on I/O. Preferable SQL based, but not necessarily.

Thanks

What are RRD graphs?

What RRDtool does. RRDtool is the OpenSource industry standard, high performance data logging and graphing system for time series data. RRDtool can be easily integrated in shell scripts, perl, python, ruby, lua or tcl applications.

There are some time series databases which have high availability and/or scalability as goals.

Maybe have a look at

rrdcached, a caching layer on top of rrd
whisper, the database engine behind graphite
opentsdb is a distributed, scalable Time Series Database (TSDB) written on top of HBase
reconnoiter although its focus is more on monitoring