Columnar Storage

You’re going to hear a lot about columnar storage formats in the next few months, as a variety of distributed execution engines are beginning to consider them for their IO efficiency, and the optimisations that they open up for query execution. In this post, I’ll explain why we care so much about IO efficiency and show how columnar storage – which is a simple idea – can drastically improve performance for certain workloads.

Caveat: This is a personal, general research summary post, and as usual doesn’t neccessarily reflect our thinking at Cloudera about columnar storage.

Disks are still the major bottleneck in query execution over large datasets. Even a machine with twelve disks running in parallel (for an aggregate bandwidth of north of 1GB/s) can’t keep all the cores busy; running a query against memory-cached data can get tens of GB/s of throughput. IO bandwidth matters. Therefore, the best thing an engineer can do to improve the performance of disk-based query engines (like RDBMs and Impala) usually is to improve the performance of reading bytes from disk. This can mean decreasing the latency (for small queries where the time to find the data to read might dominate), but most usually this means improving the effective throughput of reads from disk.

The traditional way to improve disk bandwidth has been to wait, and allow disks to get faster. However, disks are not getting faster very quickly (having settled at roughly 100 MB/s, with ~12 disks per server), and SSDs can’t yet achieve the storage density to be directly competitive with HDDs on a per-server basis.

The other way to improve disk performance is to maximise the ratio of ‘useful’ bytes read to total bytes read. The idea is not to read more data than is absolutely necessary to serve a query, so the useful bandwidth realised is increased without actually improving the performance of the IO subsystem. Enter columnar storage, a principle for file format design that aims to do exactly that for query engines that deal with record-based data.

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