A question that often crops up is “Can I use local storage”, the answer is “It depends”. Kubernetes is essentially a container scheduler at its most basic and fundamental level. The ‘Pod’ is the unit of scheduling, containers in the same pod share the same life cycle and always run on the same node. For stateless pods life is reasonably simple and straight forward, for state-full pods, life is a bit more nuanced. If for any reason a node fails, the pods that ran on that node have to be rescheduled to run on a working node, and their storage needs to follow them. This involves un-mounting the volume from the failed node and then mounting it on the node the pod(s) are rescheduled to run on. With basic vanilla hyper-converged storage, i.e. storage and compute in the same chassis, this will ultimately lead to scheduling problems. However, software defined solutions exist that enable this kind of infrastructure to be turned into a storage cluster which allows state to follow pods around the cluster. Some people automatically associated HDFS with local storage, the reason for this is probably because “Back in the day”, the most cost efficient way for Google to scale out its infrastructure was via commodity servers with local disks.
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