huginn created HBASE-30391:
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Summary: Reduce array copying when expanding ByteArrayOutputStream
Key: HBASE-30391
URL: https://issues.apache.org/jira/browse/HBASE-30391
Project: HBase
Issue Type: Improvement
Components: HFile
Affects Versions: 2.4.11
Reporter: huginn
## What happens
The custom ByteArrayOutputStream copies the entire backing array when it
expands, including unused capacity.
## When it happens
This occurs when a write exceeds the current buffer capacity and the stream
allocates a larger backing array.
## Impact
The unnecessary copy increases CPU work and temporary heap memory traffic,
especially for buffers whose logical content is much smaller than their
capacity.
## Root cause
In ByteArrayOutputStream.checkSizeAndGrow, the expansion path copies buf.length
bytes even though only pos bytes contain valid stream data.
## Proposed fix
Copy only the bytes already written to the stream (pos) when expanding the
backing array. This preserves the stream contents while avoiding copies of
unused capacity.
## Reproduction
The behavior can be observed by writing data in increments that trigger buffer
expansion and profiling the resulting array-copy work. Testing evidence will be
added by the reporter.{color:red}着色文本{color}
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