[GitHub] [spark] steveloughran commented on a change in pull request #31495: [SPARK-34383][SS] Optimize WAL commit phase via reducing cost of filesystem operations

[GitBox <gi...@apache.org>]

steveloughran commented on a change in pull request #31495:
URL: https://github.com/apache/spark/pull/31495#discussion_r571948537



##########
File path: sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/HDFSMetadataLog.scala
##########
@@ -239,18 +239,35 @@ class HDFSMetadataLog[T <: AnyRef : ClassTag](sparkSession: SparkSession, path:
       .reverse
   }
 
+  private var lastPurgedBatchId: Long = -1L
+
   /**
    * Removes all the log entry earlier than thresholdBatchId (exclusive).
    */
   override def purge(thresholdBatchId: Long): Unit = {
-    val batchIds = fileManager.list(metadataPath, batchFilesFilter)
-      .map(f => pathToBatchId(f.getPath))
-
-    for (batchId <- batchIds if batchId < thresholdBatchId) {
-      val path = batchIdToPath(batchId)
-      fileManager.delete(path)
-      logTrace(s"Removed metadata log file: $path")
+    val possibleTargetBatchIds = (lastPurgedBatchId + 1 until thresholdBatchId)
+    if (possibleTargetBatchIds.length <= 3) {
+      // avoid using list if we only need to purge at most 3 elements
+      possibleTargetBatchIds.foreach { batchId =>
+        val path = batchIdToPath(batchId)
+        if (fileManager.exists(path)) {

Review comment:
       delete() is a no-op if the file doesn't exist; it always has to do the check to see if its there (and whether a file/dir), so wrapping it with another check is superfluous. Azure abfs will suffer here, HDFS less, but still two RPCs with one block for NN read access, one for NN r/w. I don't know anything about HDFS NN locking so can't comment there




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