André Frimberger created HDFS-11311:
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Summary: HDFS fsck continues to report all blocks present when
DataNode is restarted with empty data directories
Key: HDFS-11311
URL: https://issues.apache.org/jira/browse/HDFS-11311
Project: Hadoop HDFS
Issue Type: Bug
Components: namenode
Affects Versions: 3.0.0-alpha1, 2.7.3
Reporter: André Frimberger
During cluster maintenance, we had to change parameters of the underlying disk
filesystem and we stopped the DataNode, reformatted all of its data directories
and started the DataNode again in under 10 minutes with no data and only the
{{VERSION}} file present. Running fsck afterwards reports that all blocks are
fully replicated, which does not reflect the true state of HDFS. If an
administrator trusts {{fsck}} and continues to replace further DataNodes, *data
will be lost!*
Steps to reproduce:
1. Shutdown DataNode
2. Remove all BlockPools from all data directories (only {{VERSION}} file is
present)
3. Startup DataNode in under 10.5 minutes
4. Run {{hdfs fsck /}}
*Actual result:* Average replication is falsely shown as 3.0
*Expected result:* Average replication factor is < 3.0
*Workaround:* Trigger a block report with {{hdfs dfsadmin -triggerBlockReport
$dn_host:$ipc_port}}
*Cause:* The first block report is handled differently by NameNode and only
added blocks are respected. This behaviour was introduced in HDFS-7980 for
performance reasons. But is applied too widely and in our case data can be lost.
*Fix:* We suggest using stricter conditions on applying
{{processFirstBlockReport}} in {{BlockManager:processReport()}}:
Change
{code}
if (storageInfo.getBlockReportCount() == 0) {
// The first block report can be processed a lot more efficiently than
// ordinary block reports. This shortens restart times.
processFirstBlockReport(storageInfo, newReport);
} else {
invalidatedBlocks = processReport(storageInfo, newReport);
}
{code}
to
{code}
if (storageInfo.getBlockReportCount() == 0 && storageInfo.getState() !=
State.FAILED && storageInfo.numBlocks() > 0) {
// The first block report can be processed a lot more efficiently than
// ordinary block reports. This shortens restart times.
processFirstBlockReport(storageInfo, newReport);
} else {
invalidatedBlocks = processReport(storageInfo, newReport);
}
{code}
In case the DataNode reports no blocks for a data directory, it might be a new
DataNode or the data directory may have been emptied for whatever reason
(offline replacement of storage, reformatting of data disk, etc.). In either
case, the changes should be reflected in the output of {{fsck}} in less than 6
hours to prevent data loss due to misleading output.
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