ONTAP NVRAM (Non-Volatile RAM), or NVMEM in modern systems, serves as a high-performance journal for all incoming write operations. When a host sends a write request, ONTAP logs the data to NVRAM and immediately sends an acknowledgment back to the host. This data is considered "uncommitted" until it is flushed to the persistent disk-based storage during a consistency point. This logging mechanism significantly improves write latency. To ensure the integrity of these uncommitted writes in the event of a power failure or unplanned shutdown, the NVRAM is non-volatile, a feature traditionally provided by an onboard battery that maintains power to the memory.

A. ONTAP NVRAM is used as read cache.

Incorrect. ONTAP uses system memory (RAM) as its primary read cache (buffer cache) and can use optional Flash Cache or Flash Pool technologies as a secondary read cache.

C. ONTAP NVRAM is used as write cache.

Incorrect. While its function is similar to a write cache, its role is more accurately described as a write log or journal. Option B, "stores uncommitted writes," is a more precise description of this logging function within the WAFL file system.

1. NetApp ONTAP Concepts Documentation

"How the WAFL file system works": "All write requests go to a dedicated

high-performance

nonvolatile storage (NVRAM or NVMEM) on the storage controller. They are treated as logged writes. After the writes are logged

the client is sent a confirmation." This source validates that NVRAM stores uncommitted writes (logged writes).

2. NetApp Support Knowledge Base

Article ID: 1011991

"What is the purpose of NVRAM?": "NVRAM is battery-backed memory that is present on all filers. Its purpose is to log incoming NFS/CIFS/iSCSI/FCP write requests... In the event of a power failure

the contents of NVRAM are preserved by the battery." This source validates that NVRAM is battery-backed and stores incoming (uncommitted) writes.

3. NetApp AFF and FAS Documentation

"AFF A900 and FAS9500 Systems Documentation Center"

Section: "Replace the NVMEM battery": "The NVMEM battery enables the controller module to save the contents of memory to flash in the event of a loss of power." This hardware-level documentation confirms the use of a battery to ensure non-volatility.