DirectIOIT requires a filesystem that supports Direct IO.

[ChrisHegarty]

DirectIOIT requires a filesystem that supports Direct IO.

The Elasticsearch CI builds and tests on a tmpfs, /dev/shm/bk, which does not support O_DIRECT.

relates #128370

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[ChrisHegarty]

Without this change we encounter failures like:

 Caused by: java.nio.file.FileSystemException: /dev/shm/bk/chegar/elasticsearch/server/build/testrun/internalClusterTest/temp/org.elasticsearch.index.store.DirectIOIT_6D6
A5FF9F7D9F475-001/tempDir-002/node_s3/d1/indices/LfI708mWQmmunpqa3_2VFw/9/
index/_0_ES818BinaryQuantizedVectorsFormat_0.vec: Invalid argument
        at sun.nio.fs.UnixException.translateToIOException(UnixException.java:100) ~[?:?]
        at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:106) ~[?:?]
        at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:111) ~[?:?]
        at sun.nio.fs.UnixFileSystemProvider.newFileChannel(UnixFileSystemProvider.java:213) ~[?:?]
        at java.nio.channels.FileChannel.open(FileChannel.java:301) ~[?:?]
        at java.nio.channels.FileChannel.open(FileChannel.java:353) ~[?:?]
        at org.apache.lucene.misc.store.DirectIODirectory$DirectIOIndexInput.<init>(DirectIODirectory.java:318) ~[lucene-misc-10.2.1.jar:10.2.1 1b2451b5f435ad2c41a8b6b0f0d6a59b24ff9ad2 - 2025-04-25 16:50:53]
        at org.apache.lucene.misc.store.DirectIODirectory.openInput(DirectIODirectory.java:173) ~[lucene-misc-10.2.1.jar:10.2.1 1b2451b5f435ad2c41a8b6b0f0d6a59b24ff9ad2 - 2025-04-25 16:50:53]
        at org.elasticsearch.index.store.FsDirectoryFactory$HybridDirectory.openInputDirect(FsDirectoryFactory.java:199) ~[main/:?]
        at org.elasticsearch.index.codec.vectors.es818.DirectIOLucene99FlatVectorsReader.openDataInput(DirectIOLucene99FlatVectorsReader.java:131) ~[main/:?]
...

[thecoop]

Shouldn't this check be within Elasticsearch? ES should work regardless of what the underlying file system is, using direct io opportunistically

[ChrisHegarty]

While the Elasticsearch CI builds and tests on a tmpfs, /dev/shm/bk, which does not support O_DIRECT, there are "periodic" builds/tests that use a "real" file-system. So there is coverage for directIO in "the system", just not on every PR.