What is the purpose of the OpLog?

The Nutanix OpLog is a core component of the Distributed Storage Fabric (DSF) that functions as a persistent write buffer. It is structured as a staging area on low-latency SSDs within each Controller VM (CVM). The OpLog's primary purpose is to absorb bursts of random write I/O from user VMs. Once a write is committed to the local OpLog and replicated to another CVM's OpLog in the cluster, it is acknowledged to the hypervisor. This process provides low-latency write performance. The OpLog then coalesces these random writes and drains them sequentially to the persistent storage tier, the Extent Store, optimizing backend disk performance.

B. Persistent data storage: The Extent Store is the long-term persistent data storage tier, whereas the OpLog is a temporary write staging area.
C. Global metadata: Cluster-wide metadata is managed by Medusa, a distributed key-value store, and Zookeeper, not the OpLog.
D. Dynamic read cache: The Unified Cache is the component responsible for caching read data; the OpLog is exclusively for write operations.

1. The Nutanix Bible: In the "IO Path Overview" section, the OpLog is explicitly defined: "The Nutanix architecture has a persistent write buffer, similar to a file system journal, called the OpLog... The OpLog is built on top of a staging area of SSD drives." This confirms its role as a persistent write buffer.

2. Nutanix University, Nutanix Hybrid Cloud Fundamentals Course Guide v6.5: In Module 3, "Distributed Storage Fabric," the I/O path is detailed. The guide states, "For random writes, the data is sent to the OpLog... The OpLog acts as a write cache on SSD." (Section: I/O Operations). This source verifies its function as a write buffer on SSD.

3. Nutanix Acropolis Advanced Administration and Performance Management Guide (AOS 6.5): In the "Nutanix I/O Path" chapter, the guide explains, "For most random writes, the Nutanix cluster writes the data to the oplog, which is a fast, persistent write buffer on an SSD." This directly supports the answer.