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From "Cosmin Lehene (JIRA)" <j...@apache.org>
Subject [jira] Created: (HBASE-1876) DroppedSnapshotException when flushing memstore after a datanode dies
Date Wed, 30 Sep 2009 16:49:23 GMT
DroppedSnapshotException when flushing memstore after a datanode dies

                 Key: HBASE-1876
                 URL: https://issues.apache.org/jira/browse/HBASE-1876
             Project: Hadoop HBase
          Issue Type: Bug
          Components: regionserver
    Affects Versions: 0.20.0
            Reporter: Cosmin Lehene
            Priority: Critical
             Fix For: 0.20.1

A dead datanode in the cluster can lead to multiple HRegionServer failures and corrupted data.
The HRegionServer failures can be reproduced  consistently on a 7 machines cluster with approx
2000 regions.

Steps to reproduce

The easiest and safest way is to reproduce it for the .META. table, however it will work with
any table. 

Locate a datanode that stores the .META. files and kill -9 it. 
In order to get multiple writes to the .META. table bring up or shut down a region server
this will eventually cause a flush on the memstore

2009-09-25 09:26:17,775 DEBUG org.apache.hadoop.hbase.regionserver.HRegion: Flush requested
on .META.,demo__assets,asset_283132172,1252898166036,1253265069920
2009-09-25 09:26:17,775 DEBUG org.apache.hadoop.hbase.regionserver.HRegion: Started memstore
flush for region .META.,demo__assets,asset_283132172,1252898166036,1253265069920. Current
region memstore si
ze 16.3k
2009-09-25 09:26:17,791 INFO org.apache.hadoop.hdfs.DFSClient: Exception in createBlockOutputStream
java.io.IOException: Bad connect ack with firstBadLink
2009-09-25 09:26:17,791 INFO org.apache.hadoop.hdfs.DFSClient: Abandoning block blk_-8767099282771605606_176852

The DFSClient will retry for 3 times, but there's a high chance it will try on the same failed
datanode (it takes around 10 minutes for dead datanode to be removed from cluster)

2009-09-25 09:26:41,810 WARN org.apache.hadoop.hdfs.DFSClient: DataStreamer Exception: java.io.IOException:
Unable to create new block.
        at org.apache.hadoop.hdfs.DFSClient$DFSOutputStream.nextBlockOutputStream(DFSClient.java:2814)
        at org.apache.hadoop.hdfs.DFSClient$DFSOutputStream.access$2000(DFSClient.java:2078)
        at org.apache.hadoop.hdfs.DFSClient$DFSOutputStream$DataStreamer.run(DFSClient.java:2264)

2009-09-25 09:26:41,810 WARN org.apache.hadoop.hdfs.DFSClient: Error Recovery for block blk_5317304716016587434_176852
bad datanode[2] nodes == null
2009-09-25 09:26:41,810 WARN org.apache.hadoop.hdfs.DFSClient: Could not get block locations.
Source file "/hbase/.META./225980069/info/5573114819456511457" - Aborting...
2009-09-25 09:26:41,810 FATAL org.apache.hadoop.hbase.regionserver.MemStoreFlusher: Replay
of hlog required. Forcing server shutdown
org.apache.hadoop.hbase.DroppedSnapshotException: region: .META.,demo__assets,asset_283132172,1252898166036,1253265069920
        at org.apache.hadoop.hbase.regionserver.HRegion.internalFlushcache(HRegion.java:942)
        at org.apache.hadoop.hbase.regionserver.HRegion.flushcache(HRegion.java:835)
        at org.apache.hadoop.hbase.regionserver.MemStoreFlusher.flushRegion(MemStoreFlusher.java:241)
        at org.apache.hadoop.hbase.regionserver.MemStoreFlusher.run(MemStoreFlusher.java:149)
Caused by: java.io.IOException: Bad connect ack with firstBadLink
        at org.apache.hadoop.hdfs.DFSClient$DFSOutputStream.createBlockOutputStream(DFSClient.java:2872)
        at org.apache.hadoop.hdfs.DFSClient$DFSOutputStream.nextBlockOutputStream(DFSClient.java:2795)
        at org.apache.hadoop.hdfs.DFSClient$DFSOutputStream.access$2000(DFSClient.java:2078)
        at org.apache.hadoop.hdfs.DFSClient$DFSOutputStream$DataStreamer.run(DFSClient.java:2264)

After the HRegionServer shuts down itself the regions will be reassigned however you might
hit this 

2009-09-26 08:04:23,646 INFO org.apache.hadoop.hbase.regionserver.HRegionServer: Worker: MSG_REGION_OPEN:
2009-09-26 08:04:23,684 WARN org.apache.hadoop.hbase.regionserver.Store: Skipping hdfs://b0:9000/hbase/.META./225980069/historian/1432202951743803786
because its empty. HBASE-646 DATA LOSS?
2009-09-26 08:04:23,776 INFO org.apache.hadoop.hbase.regionserver.HRegion: region .META.,demo__assets,asset_283132172,1252898166036,1253265069920/225980069
available; sequence id is 1331458484

We ended up with corrupted data in .META. "info:server" after master got confirmation that
it was updated from the HRegionServer that got DroppedSnapshotException

Since after a cluster restart server:info will be correct, .META. is safer to test with. Also
to detect data corruption you can just scan .META. get the start key for each region and attempt
to retrieve it from the corresponding table. If .META. is corrupted you get a NotServingRegionException.

This issue is related to https://issues.apache.org/jira/browse/HDFS-630 

I attached a patch for HDFS-630 https://issues.apache.org/jira/secure/attachment/12420919/HDFS-630.patch
that fixes this problem. 

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