[jira] [Assigned] (CASSANDRA-21487) trickle_fsync is silently ignored for compressed SSTables

["Sam Lightfoot (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/CASSANDRA-21487?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Sam Lightfoot reassigned CASSANDRA-21487:
-----------------------------------------

    Assignee: Sam Lightfoot

> trickle_fsync is silently ignored for compressed SSTables
> ---------------------------------------------------------
>
>                 Key: CASSANDRA-21487
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-21487
>             Project: Apache Cassandra
>          Issue Type: Bug
>          Components: Local/SSTable
>            Reporter: Sam Lightfoot
>            Assignee: Sam Lightfoot
>            Priority: Normal
>
> h2. What's wrong
> {{trickle_fsync: true}} is meant to fsync a large sequential write at intervals, so a flush
> trickles its dirty pages to disk instead of dumping them all at once and spiking read latency.
> On compressed SSTables it does nothing. Compression is the default, so the setting has been silently ignored on the data file of every flush and every buffered compaction. No error, no log line - it just doesn't work.
> h2. Why
> The compressed writer throws the setting away. {{CompressedSequentialWriter.buildOption()}} rebuilds the writer's options and copies only three of them:
> {code:java}
> return SequentialWriterOption.newBuilder()
>                              .bufferSize(parameters.chunkLength())
>                              .bufferType(parameters.getSstableCompressor().preferredBufferType())
>                              .finishOnClose(option.finishOnClose())   // trickleFsync + interval lost here
>                              .build();
> {code}
> {{trickleFsync}} and {{trickleFsyncByteInterval}} never make it through, so the rebuilt option defaults to {{{}trickleFsync = false{}}}. The trickle branch in {{SequentialWriter.doFlush}}
> ({{{}if (option.trickleFsync()) …{}}}) never runs, and no interval fsync ever fires.
> {{IOOptions}} and {{BigTableWriter}} set the options correctly; only the compressed writer drops them. Commit {{fb221095cb}} (June 2016) moved trickle config into the passed-in option but forgot this path. It has been broken ever since.
> h2. What still works
> The primary index (plain {{{}SequentialWriter{}}}) and uncompressed data files
> ({{{}ChecksummedSequentialWriter{}}}) pass the option through untouched, so trickle works there. But the index is tiny; the data file is where the bytes and the writeback storm are.
> h2. Fix
> Pass the two fields through:
> {code:java}
>     .trickleFsync(option.trickleFsync())
>     .trickleFsyncByteInterval(option.trickleFsyncByteInterval())
> {code}
> The interval counter in {{doFlush}} already tracks uncompressed bytes correctly; only the enable flag was missing.
> h2. How it was found
> Found while benchmarking flush write-pacing: {{trickle_fsync}} on and off gave an identical dirty window, which led here. Fix this on its own. The proposed {{trickle_fsync_mode: writeback }}improvement is separate work that adds a third field to propagate through the same method.



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