Hadoop 19641: [ABFS][ReadAheadV2] First Read should bypass ReadBufferManager

[anujmodi2021]

Description of PR

JIRA: https://issues.apache.org/jira/browse/HADOOP-19641
We have observed this across multiple workload runs that when we start reading data from input stream. The first read which came to input stream has to be read synchronously even if we trigger prefetch request for that particular offset. Most of the times we end up doing extra work of checking if the prefetch is trigerred, removing prefetch from the pending queue and go ahead to do a direct remote read in workload thread itself.

To avoid all this overhead, we will always bypass read ahead for the very first read of each input stream and trigger read aheads for second read onwards.

How was this patch tested?

TBA

[Copilot]

Pull Request Overview

This PR optimizes the read-ahead behavior in ABFS input streams by bypassing the ReadBufferManager for the first read operation, reducing unnecessary overhead when the read pattern is not yet established.

• Introduces a bypass mechanism for the first read to avoid synchronous prefetch overhead
• Modifies the number of read-ahead requests from configurable depth to 1 for the first read
• Updates existing tests to account for the new first-read bypass behavior

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
AbfsInputStream.java	Implements first-read bypass logic by setting numReadAheads to 1 and adds isFirstRead() method
ReadBufferManagerV1.java	Prevents removal of prefetch buffers from queue when triggered by first read
TestAbfsInputStream.java	Updates test expectations and mocking to accommodate the new first-read behavior

[Copilot]

[nitpick] The ternary operator lacks spaces around the ? operator, which deviates from Java coding conventions. It should be firstRead ? 1 : this.readAheadQueueDepth.

Suggested change

	int numReadAheads = firstRead? 1 : this.readAheadQueueDepth;
	int numReadAheads = firstRead ? 1 : this.readAheadQueueDepth;

[Copilot]

The log message mentions 'read size of {}' but logs bufferSize instead of the actual read size (b.length). This could be misleading when debugging read operations.

Suggested change

	LOG.debug("Sequential read with read size of {} and read ahead enabled", bufferSize);
	LOG.debug("Sequential read with read size of {} and read ahead enabled", b.length);