Andrey Yarovoy created HDDS-17717:
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Summary: OzoneFSInputStream should implement CanSetDropBehind as a
no-op
Key: HDDS-17717
URL: https://issues.apache.org/jira/browse/HDDS-17717
Project: Apache Ozone
Issue Type: Improvement
Components: Ozone Filesystem
Reporter: Andrey Yarovoy
*Description*
`OzoneFSInputStream` implements `ByteBufferReadable`, `CanUnbuffer` and
`ByteBufferPositionedReadable`, but not `CanSetDropBehind`. Hadoop's
`FSDataInputStream.setDropBehind` casts the wrapped stream to
`CanSetDropBehind`, and on `ClassCastException` throws
`UnsupportedOperationException`. Both exceptions fill in a stack trace.
HBase calls `setDropBehind` on every stream it opens
(`FSDataInputStreamWrapper.setStreamOptions`) and swallows the exception. On
HBase every
store-file reopen (`StoreScanner.trySwitchToStreamRead`) opens two streams, so
the exception is thrown twice per reopen per store file.
*Evidence (HBase YCSB on EC, RegionServer async-profiler JFR, 6×60s per arm)*
`FSDataInputStream.setDropBehind` → `UnsupportedOperationException.<init>` →
`Throwable.fillInStackTrace` costs **0.75% (stream read ON) and
0.96% (OFF)** of RS CPU. That is pure exception cost; the call has no effect.
*Proposed fix*
{code:java}
public class OzoneFSInputStream extends FSInputStream
implements ByteBufferReadable, CanUnbuffer, ByteBufferPositionedReadable,
CanSetDropBehind {
/** Drop-behind is a page-cache hint for the local client; Ozone reads go to
datanodes, so it is accepted and ignored. */
@Override
public void setDropBehind(Boolean dropCache) {
}
}
{code}
- Same for the o3fs path; both share `OzoneFSInputStream`.
- `CapableOzoneFSInputStream.hasCapability(StreamCapabilities.DROPBEHIND)`
keeps returning `false`. The stream accepts the hint but does not
implement it, so it should not advertise the capability. Callers that probe
capabilities first see no change.
*Rejected alternative*
Forward the hint to the datanode (`fadvise`-style, like HDFS). That would be a
protocol and feature change; the cost here is the exception.
*Compatibility / side effects*
`setDropBehind` stops throwing `UnsupportedOperationException`. Callers that
treat the exception as "unsupported" (HBase logs and ignores it)
now see a silent no-op. No wire change.
*Tests*
- Unit: `setDropBehind(true/false/null)` on an ofs and an o3fs
`FSDataInputStream` does not throw; `hasCapability("dropbehind")` stays false.
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