Alluxio with/without HDFS

Reply from Alluxio maintainer.

First of all, Alluxio is not a replacement for HDFS. Instead, it is a new abstraction layer on top of other distributed/cloud storage systems including HDFS, S3, Azure Object Store and other possible choices. In your case, if you data is already in HDFS, you will perhaps still keep HDFS as the persistent data layer for Alluxio.

The typical scenarios users put Alluxio in the picture and see significant benefits include:

• Your physical data is not located with your compute. E.g., your bigdata engine is reading data from S3 or other object storage. In this case, by deploying Alluxio with compute nodes, one can make Alluxio work as a filesystem level cache to avoid fetching data across network repeatedly. See http://www.alluxio.org/overview/remote-data-acceleration
• You are managing multiple storages and want to expose a single data access layer to simplify the management. E.g., one can "mount" multiple S3/ buckets into one Alluxio deployment so they appear as different directories under the same namespace. See http://www.alluxio.org/overview/storage-unification

Regarding your original performance question. The answer is, it depends. If your HDFS is remote from compute, you would expect a good performance gain. I also saw cases when HDFS is bottlenecked, Alluxio may also help to reduce the load and provides good SLA for certain mission-critical jobs.