[HADOOP-14512] WASB atomic rename should not throw exception if the file is neither in src nor in dst when doing the rename

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.8.0
• Fix Version/s: 3.0.0-alpha4, 2.8.2
• Component/s: fs/azure
• Hadoop Flags: Reviewed
• Assignee: Duo Xu
• Reporter: Duo Xu
• Created: 08/Jun/17 22:57
• Updated: 15/Jun/17 20:36
• Resolved: 09/Jun/17 20:09

Description

During atomic rename operation, WASB creates a rename pending json file to document which files need to be renamed and the destination. Then WASB will read this file and rename all the files one by one.

There is a recent customer incident in HBase showing a potential bug in the atomic rename implementation,

For example, below is a rename pending json file,

{
  FormatVersion: "1.0",
  OperationUTCTime: "2017-04-29 06:08:57.465",
  OldFolderName: "hbase\/data\/default\/abc",
  NewFolderName: "hbase\/.tmp\/data\/default\/abc",
  FileList: [
    ".tabledesc",
    ".tabledesc\/.tableinfo.0000000001",
    ".tmp",
    "08e698e0b7d4132c0456b16dcf3772af",
    "08e698e0b7d4132c0456b16dcf3772af\/.regioninfo",
    "08e698e0b7d4132c0456b16dcf3772af\/0\/617294e0737e4d37920e1609cf539a83",
    "08e698e0b7d4132c0456b16dcf3772af\/recovered.edits\/185.seqid",
    "08e698e0b7d4132c0456b16dcf3772af\/.regioninfo",
    "08e698e0b7d4132c0456b16dcf3772af\/0",
 "08e698e0b7d4132c0456b16dcf3772af\/0\/617294e0737e4d37920e1609cf539a83",
    "08e698e0b7d4132c0456b16dcf3772af\/recovered.edits",
    "08e698e0b7d4132c0456b16dcf3772af\/recovered.edits\/185.seqid"
  ]
}

When HBase regionserver process (underlying is using WASB driver) was renaming "08e698e0b7d4132c0456b16dcf3772af\/.regioninfo", the regionserver process crashed or the VM got rebooted due to system maintenence. When the regionserver process started running again, it found the rename pending json file and tried to redo the rename operation.

However, when it read the first file ".tabledesc" in the file list, it could not find this file in src folder and it also could not find the file in destination folder. It could not find it in src folder because the file had already been renamed/moved to the destination folder. It could not find it in destination folder because when HBase starts, it will clean up all the files under /hbase/.tmp.

The current implementation will throw exceptions saying

else {
        throw new IOException(
            "Attempting to complete rename of file " + srcKey + "/" + fileName
            + " during folder rename redo, and file was not found in source "
            + "or destination.");
      }

This will cause HBase HMaster initialization failure and restart HMaster will not work because the same exception will throw again.

My proposal is that if during the redo, WASB finds a file not in src and not in dst, WASB should just skip this file and process the next file rather than throw the error and let user manually fix it. Reasons are

1. Since the rename pending json file contains file A, if the file A is not in src, it must have been renamed.
2. if the file A is not in src and not in dst, the upper layer service must have removed it. One thing to note is that during the atomic rename, the folder is locked. So the only situation the file gets deleted is when VM reboots or service process crashes. When service process restarts, there might be some operations happening before the atomic rename redo, like the HBase example above.