Yannick Welsch created LUCENE-9264:
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Summary: Remove SimpleFSDirectory in favor of NIOFsDirectory
Key: LUCENE-9264
URL: https://issues.apache.org/jira/browse/LUCENE-9264
Project: Lucene - Core
Issue Type: Improvement
Reporter: Yannick Welsch
{{SimpleFSDirectory}} looks to duplicate what's already offered by
{{NIOFsDirectory}}. The only difference is that {{SimpleFSDirectory}} is using
non-positional reads on the {{FileChannel}} (i.e., reads that are stateful,
changing the current position), and {{SimpleFSDirectory}} therefore has to
externally synchronize access to the read method.
On Windows, positional reads are not supported, which is why {{FileChannel}} is
already internally using synchronization to guarantee only access by one thread
at a time for positional reads (see {{read(ByteBuffer dst, long position)}} in
{{FileChannelImpl}}, and {{FileDispatcher.needsPositionLock}}, which returns
true on Windows) and the JDK implementation for Windows is emulating positional
reads by using non-positional ones, see
[http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/windows/native/sun/nio/ch/FileDispatcherImpl.c#l139].
This means that on Windows, there should be no difference between
{{NIOFsDirectory}} and {{SimpleFSDirectory}} in terms of performance (it should
be equally poor as both implementations only allow one thread at a time to
read). On Linux/Mac, {{NIOFsDirectory}} is superior to {{SimpleFSDirectory}},
however, as positional reads (pread) can be done concurrently.
My proposal is to remove {{SimpleFSDirectory}} and replace its uses with
{{NIOFsDirectory}}, given how similar these two directory implementations are
({{SimpleFSDirectory}} isn't really simpler).
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