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From "Andrei Dragomir (JIRA)" <j...@apache.org>
Subject [jira] Created: (HBASE-2176) HRegionInfo reported empty on regions in meta, leading to them being deleted, although the regions contain data and exist
Date Fri, 29 Jan 2010 20:32:34 GMT
HRegionInfo reported empty on regions in meta, leading to them being deleted, although the
regions contain data and exist
-------------------------------------------------------------------------------------------------------------------------

                 Key: HBASE-2176
                 URL: https://issues.apache.org/jira/browse/HBASE-2176
             Project: Hadoop HBase
          Issue Type: Bug
    Affects Versions: 0.21.0
            Reporter: Andrei Dragomir
            Priority: Critical


We ran some tests on our cluster, and getting back reports about WrongRegionException, on
some rows. After looking at the data, we see that we have "gaps" between regions, like this:

{noformat}
demo__users,user_8949795897,1264089193398  l2:60030  736660864  user_8949795897  user_8950697145
<- end key
demo__users,user_8953502603,1263992844343  l5:60030  593335873  user_8953502603 <- should
be star key here   user_8956071605
{noformat}

Fact: we had 28 regions that were reported with empty HRegionInfo, and deleted from .META..


Fact: we recovered our data entirely, without any issues, by running the .META. restore script
from table contents (bin/add_table.rb)

Fact: on our regionservers, we have three days with no logs. To the best of our knowledge,
the machines were not rebooted, the processes were running. During these three days, on the
master, the only entry in the logs (repeated), every second, is a .META. scan:

{noformat}
2010-01-23 00:01:27,816 INFO org.apache.hadoop.hbase.master.BaseScanner: RegionManager.rootScanner
scan of 1 row(s) of meta region {server: 10.72.135.7:60020, regionname: -ROOT-,,0, startKey:
<>} complete
2010-01-23 00:01:34,413 INFO org.apache.hadoop.hbase.master.ServerManager: 6 region servers,
0 dead, average load 1113.6666666666667
2010-01-23 00:02:23,645 INFO org.apache.hadoop.hbase.master.BaseScanner: RegionManager.metaScanner
scanning meta region {server: 10.72.135.10:60020, regionname: .META.,,1, startKey: <>}
2010-01-23 00:02:26,002 INFO org.apache.hadoop.hbase.master.BaseScanner: RegionManager.metaScanner
scan of 6679 row(s) of meta region {server: 10.72.135.10:60020, regionname: .META.,,1, startKey:
<>} complete
2010-01-23 00:02:26,002 INFO org.apache.hadoop.hbase.master.BaseScanner: All 1 .META. region(s)
scanned
2010-01-23 00:02:27,821 INFO org.apache.hadoop.hbase.master.BaseScanner: RegionManager.rootScanner
scanning meta region {server: 10.72.135.7:60020, regionname: -ROOT-,,0, startKey: <>}
.......................................................
{noformat}

In the master logs, we see a pretty normal evolution: region r0 is split into r1 and r2. Now,
r1 exists and is good, r2 does not exist in .META. anymore, because it was reported as having
empty HRegionInfo. The only thing in the master logs that is weird is that the message about
updating the region in meta comes up twice:

{noformat}
2010-01-27 22:46:45,007 INFO org.apache.hadoop.hbase.master.RegionServerOperation: demo__users,user_8950697145,1264089193398
open on 10.72.135.7:60020
2010-01-27 22:46:45,010 INFO org.apache.hadoop.hbase.master.RegionServerOperation: Updated
row demo__users,user_8950697145,1264089193398 in region .META.,,1 with startcode=1264661019484,
server=10.72.135.7:60020
2010-01-27 22:46:45,010 INFO org.apache.hadoop.hbase.master.RegionServerOperation: demo__users,user_8950697145,1264089193398
open on 10.72.135.7:60020
2010-01-27 22:46:45,012 INFO org.apache.hadoop.hbase.master.RegionServerOperation: Updated
row demo__users,user_8950697145,1264089193398 in region .META.,,1 with startcode=1264661019484,
server=10.72.135.7:60020
{noformat}

Attached you will find the entire forensics work, with explanations, in a text file. 

Suppositions:

Our entire cluster was in a really weird state. All the regionservers are missing logs for
three days, and to the best of our knowledge they were running, and in this time the master
has ONLY .META. scan messages, every second, reporting 6 regionservers live, out of 7 total.


Also, during this time, we get filesystem closed messages on a regionservers with one of the
missing regions. This is after the gap in the logs. 

How we suppose the data in .META. was lost

1. Race conditions in ServerManager / RegionManager. In our logs, we have about 3 or 4 CME,
in these classes (see the attached file)
2. Data loss in HDFS. On a regionserver, we get filesystem closed messages
3. Data could not be read fro HDFS ( highly unlikely, there are no weird data read messages)
4. Race condition leading to loss of the HRegionInfo from memory, and then persisted as empty.



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