isburmistrov commented on code in PR #24079:
URL: https://github.com/apache/flink/pull/24079#discussion_r1450246875
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flink-runtime/src/main/java/org/apache/flink/runtime/state/CompressibleFSDataOutputStream.java:
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@@ -41,6 +41,10 @@ public CompressibleFSDataOutputStream(
@Override
public long getPos() throws IOException {
+ // Underlying compression involves buffering, so the only way to
report correct position is
+ // to flush the underlying stream. This lowers the effectivity of
compression, but there is
+ // no other way, since the position is often used as a split point.
+ flush();
Review Comment:
Do you think it's the best way to fix it? I think this approach has a few
drawbacks:
- As mentioned, lower efficiency of compression. Actually for some
compression algorithms this may make it useless, if not harmful (if there is
some significant amount of metadata that gets written on `flush`)
- It's somewhat strange that the result is different whether we call
`getPos()` in the process of compressing the output, or we don't. This may
later come as a big surprise for someone using `CompressibleFSDataOutputStream`.
- `Seek` operation in the corresponding `CompressibleFSDataInputStream`
doesn't make sense for any position, only for some specific positions. Which is
also not super bad, but confusing.
IMO, a better - albeit more involved - approach may be to change the
semantics of `getPos()` for `CompressibleFSDataOutputStream`. This can
represent the position in the **uncompressed** data.
Correspondingly, `seek` operation in `CompressibleFSDataInputStream` can be
implemented having this semantics in mind: one can read the
`compressingDelegate` until the desired position is reached.
This approach also doesn't have the problem of `seek` being valid for
specific positions only, which is a nice side effect.
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