[GitHub] [ozone] sodonnel commented on a change in pull request #2723: HDDS-5552. EC: Implement seek on ECBlockInputStream

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

sodonnel commented on a change in pull request #2723:
URL: https://github.com/apache/ozone/pull/2723#discussion_r727328851



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File path: hadoop-ozone/client/src/main/java/org/apache/hadoop/ozone/client/io/ECBlockInputStream.java
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@@ -255,8 +257,23 @@ public BlockID getBlockID() {
   private int readFromStream(BlockExtendedInputStream stream,
       ByteReaderStrategy strategy)
       throws IOException {
-    // Number of bytes left to read from this streams EC cell.
-    long ecLimit = ecChunkSize - (position % ecChunkSize);
+    long partialPosition = position % ecChunkSize;
+    if (seeked) {
+      // Seek on the underlying streams is performed lazily, as there is a
+      // possibility a read after a seek may only read a small amount of data.
+      // Once this block stream has been seeked, we always check the position,
+      // but in the usual case, where there are no seeks at all, we don't need
+      // to do this extra work.
+      long basePosition = (position / stripeSize) * (long)ecChunkSize;
+      long streamPosition = basePosition + partialPosition;
+      if (streamPosition != stream.getPos()) {
+        // This ECBlockInputStream has been seeked, so the underlying
+        // block stream is no longer at the correct position. Therefore we need
+        // to seek it too.
+        stream.seek(streamPosition);
+      }

Review comment:
       No, because as it stands we don't track if we have seeked all streams or not. Tracking this would add some additional complexity so I figured it was easier to just do the check each time after a block has been seeked.
   
   The usual case would not have seeks, and if we have seeked the block at all then we can pay the small overhead of checking the position on each read.




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