HDDS-9940. Use MappedByteBuffer in ChunkManagerDummyImpl

[Russole]

What changes were proposed in this pull request?

• Updated ChunkManagerDummyImpl to avoid allocating heap buffers on reads (previously ByteBuffer.allocate(len)), by creating a small backing file (default 1MB) and reading data from a reusable MappedByteBuffer.

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-9940

How was this patch tested?

All CI checks passed.
https://github.com/Russole/ozone/actions/runs/21101732901

[Gargi-jais11]

Thanks @Russole for the patch.
Left some review comments below.

[Gargi-jais11]

Here, the code uses ch.truncate(newSize) to expand the file when ch.size() < newSize, but truncate() only reduces file size - it cannot expand files.

According to Java API:> "If the given size is greater than or equal to the file's current size then the file is not modified."
When the file is smaller than newSize, truncate() does nothing, leaving the file unchanged. This causes silent failures when read requests exceed the current mapped buffer size.

1. Scenario 1: File is Smaller, Need Smaller Size (< 1MB)

Example:

• Current file: 500 KB
• Request: Read 256 KB
• newSize = max(1MB, 256KB) = 1MB (because of DEFAULT_MAP_SIZE)

Current: ch.size() = 500 KB
Need:    newSize = 1 MB
Check:   Is 500 KB < 1 MB? → YES 
Action:  ch.truncate(1 MB)
Result:  ❌ truncate() does NOTHING (can't expand)
         File stays 500 KB, but we need 1 MB!

Problem: File doesn't expand to 1 MB, mapping fails or maps wrong size.

2. Scenario 2: File is Larger, Need Smaller Size (< 1MB)

Example:

• Current file: 2 MB
• Request: Read 256 KB
• newSize = max(1MB, 256KB) = 1MB

Current: ch.size() = 2 MB
Need:    newSize = 1 MB
Check:   Is 2 MB < 1 MB? → NO ❌
Action:  Skip truncate (condition false)
Result:  File is already big enough (2 MB > 1 MB)
         But we're mapping only 1 MB, so it works!

Status: Works (but wastes space - file is 2 MB but we only use 1 MB)

We need to take care of these scenarios.

if (ch.size() < newSize) {
  // LOGIC:  Need to EXPAND file 
} else if (ch.size() > newSize) {
  // Need to SHRINK file
  ch.truncate(newSize);  // This works for shrinking!
}

[Gargi-jais11]

Add some test coverage in TestChunkManagerDummyImpl for below scenarios:

• Test when read size is exactly at current mapped size boundary
• Test when read size larger than current mapped size (triggers resize)
• Test Concurrent reads from multiple threads