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From "Liu Shaohui (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-11868) Data loss in hlog when the hdfs is unavailable
Date Mon, 01 Sep 2014 02:48:20 GMT
Liu Shaohui created HBASE-11868:

             Summary: Data loss in hlog when the hdfs is unavailable
                 Key: HBASE-11868
                 URL: https://issues.apache.org/jira/browse/HBASE-11868
             Project: HBase
          Issue Type: Bug
    Affects Versions: 0.98.5
            Reporter: Liu Shaohui
            Assignee: Liu Shaohui
            Priority: Blocker

When using the new thread model in hbase, we found a bug which may cause data loss when the
the hdfs is unavailable.

When writing wal Edits to hlog in doMiniBatchMutation of HRegion, the hlog first call appendNoSync
to write the edits to hlog and then call sync with txid. 

Assumed that the txid of current write is 10, and the syncedTillHere in hlog is 9 and the
failedTxid is 0. When  the the hdfs is unavailable, the AsyncWriter or AsyncSyncer will fail
to apend the edits or sync, then they will update the syncedTillHere to 10 and the failedTxid
to 10.

When the hlog calls the sync with txid :10, the failedTxid will nerver be checked for txid
is less than syncedTillHere.  The client thinks the write success , but the data only be writtten
to memstore not hlog. If the regionserver is down later before the memstore if flushed, the
data will be lost.
  // sync all transactions upto the specified txid
  private void syncer(long txid) throws IOException {
    synchronized (this.syncedTillHere) {
      while (this.syncedTillHere.get() < txid) {
        try {

          if (txid <= this.failedTxid.get()) {
            assert asyncIOE != null :
              "current txid is among(under) failed txids, but asyncIOE is null!";
            throw asyncIOE;
        } catch (InterruptedException e) {
          LOG.debug("interrupted while waiting for notification from AsyncNotifier");

We can fix this issue by moving the comparing of txid and failedTxid outside the while block.

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