Gargi-jais11 commented on code in PR #9649:
URL: https://github.com/apache/ozone/pull/9649#discussion_r2715401405
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hadoop-hdds/container-service/src/main/java/org/apache/hadoop/ozone/container/keyvalue/impl/ChunkManagerDummyImpl.java:
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@@ -40,6 +46,49 @@
*/
public class ChunkManagerDummyImpl implements ChunkManager {
+ private static final int DEFAULT_MAP_SIZE = 1 * 1024 * 1024; // 1MB
+
+ private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
+ private volatile MappedByteBuffer mapped;
+ private volatile int mappedSize;
+ private volatile Path backingFile;
+
+ private void ensureMapped(int minSize)
+ throws StorageContainerException {
+ if (mapped != null && mappedSize >= minSize) {
+ return;
+ }
+
+ lock.writeLock().lock();
+ try {
+ if (mapped != null && mappedSize >= minSize) {
+ return;
+ }
+
+ int newSize = Math.max(DEFAULT_MAP_SIZE, minSize);
+ if (backingFile == null) {
+ backingFile = Files.createTempFile("ozone-dummy-chunk-", ".bin");
+ backingFile.toFile().deleteOnExit();
+ }
+
+ try (FileChannel ch = FileChannel.open(backingFile,
+ StandardOpenOption.READ, StandardOpenOption.WRITE)) {
+ if (ch.size() < newSize) {
+ ch.truncate(newSize);
+ }
Review Comment:
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!
}
```
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