Manoj Govindassamy created HDFS-11552:
-----------------------------------------
Summary: Erasure Coding: Support Parity Blocks placement onto same
nodes hosting Data Blocks when DataNodes are insufficient
Key: HDFS-11552
URL: https://issues.apache.org/jira/browse/HDFS-11552
Project: Hadoop HDFS
Issue Type: Sub-task
Affects Versions: 3.0.0-alpha1
Reporter: Manoj Govindassamy
Assignee: Manoj Govindassamy
Currently, {{DFSStripedOutputStream}} verifies if the allocated block locations
are at least numDataBlocks length. That is, for the EC Policy RS-6-3-64K,
though the total needed DNs for a full EC Block Group is 9, Clients will be
able to successfully create a DFSStripedOutputStream with just 6 DNs. Moreover,
the output stream thus created with less DNs will totally ignore writing Parity
Blocks.
{code}
[Thread-5] WARN hdfs.DFSOutputStream
(DFSStripedOutputStream.java:allocateNewBlock(497)) - Failed to get block
location for parity block, index=6
[Thread-5] WARN hdfs.DFSOutputStream
(DFSStripedOutputStream.java:allocateNewBlock(497)) - Failed to get block
location for parity block, index=7
[Thread-5] WARN hdfs.DFSOutputStream
(DFSStripedOutputStream.java:allocateNewBlock(497)) - Failed to get block
location for parity block, index=8
{code}
So, upon file stream close we get the following warning message (though not
accurate) when the parity blocks are not yet written out.
{code}
INFO namenode.FSNamesystem (FSNamesystem.java:checkBlocksComplete(2726)) -
BLOCK* blk_-9223372036854775792_1002 is COMMITTED but not COMPLETE(numNodes= 0
< minimum = 6) in file /ec/test1
INFO hdfs.StateChange (FSNamesystem.java:completeFile(2679)) - DIR*
completeFile: /ec/test1 is closed by DFSClient_NONMAPREDUCE_-1900076771_17
WARN hdfs.DFSOutputStream (DFSStripedOutputStream.java:logCorruptBlocks(1117))
- Block group <1> has 3 corrupt blocks. It's at high risk of losing data.
{code}
I am not sure if there are any practical limitations in placing more blocks of
a Block Group onto the same node. At least, we can allow parity blocks co-exist
with data blocks, whenever there are insufficient DNs in the cluster. Later,
upon addition of more DataNodes, the Block Placement Policy can detect the
improper placement for such BlockGroups and can tigger EC reconstruction.
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