[HDFS-13999] Bogus missing block warning if the file is under construction when NN starts

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.6.0
• Fix Version/s: 2.7.8
• Component/s: namenode
• Target Version/s: 2.7.8
• Assignee: Wei-Chiu Chuang
• Reporter: Wei-Chiu Chuang
• Created: 16/Oct/18 20:59
• Updated: 02/Oct/19 17:13
• Resolved: 04/Dec/18 17:24

Description

We found an interesting case where web UI displays a few missing blocks, but it doesn't state which files are corrupt. What'll also happen is that fsck states the file system is healthy. This bug is similar to HDFS-10827 and HDFS-8533. 

 (See the attachment for an example)

Using Dynamometer, I was able to reproduce the bug, and realized the the "missing" blocks are actually healthy, but somehow neededReplications doesn't get updated when NN receives block reports. What's more interesting is that the files associated with the "missing" blocks are under construction when NN starts, and so after a while NN prints file recovery log.

Given that, I determined the following code is the source of bug:

BlockManager#addStoredBlock

....
   // if file is under construction, then done for now
    if (bc.isUnderConstruction()) {
      return storedBlock;
    }

which is wrong, because a file may have multiple blocks, and the first block is complete. In which case, the neededReplications structure doesn't get updated for the first block, and thus the missing block warning on the web UI. More appropriately, it should check the state of the block itself, not the file.

Fortunately, it was unintentionally fixed via HDFS-9754:

    // if block is still under construction, then done for now
    if (!storedBlock.isCompleteOrCommitted()) {
      return storedBlock;
    }

We should bring this fix into branch-2.7 too. That said, this is a harmless warning, and should go away after the under-construction-files are recovered, and the NN restarts (or force full block reports).

Kudos to Dynamometer! It would be impossible to reproduce this bug without the tool. And thanks smeng for helping with the reproduction.

Issue Links

• relates to: HDFS-9754 Avoid unnecessary getBlockCollection calls in BlockManager
• relates to: HDFS-10827 When there are unrecoverable ec block groups, Namenode Web UI shows "There are X missing blocks." but doesn't show the block names.
• relates to: HDFS-8533 Mismatch in displaying the "MissingBlock" count in fsck and in other metric reports