Consider redesign of block report processing
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Key: HDFS-1667
URL: https://issues.apache.org/jira/browse/HDFS-1667
Project: Hadoop HDFS
Issue Type: Improvement
Components: name-node
Affects Versions: 0.22.0
Reporter: Matt Foley
Assignee: Matt Foley
The current implementation of block reporting has the datanode send the entire
list of blocks resident on that node in every report, hourly.
BlockManager.processReport in the namenode runs each block report through
reportDiff() first, to build four linked lists of replicas for different
dispositions, then processes each list. During that process, every block
belonging to the node (even the unchanged blocks) are removed and re-linked in
that node's blocklist. The entire process happens under the global
FSNamesystem write lock, so there is essentially zero concurrency. It takes
about 90 milliseconds to process a single 50,000-replica block report in the
namenode, during which no other read or write activities can occur.
There are several opportunities for improvement in this design. In order of
probable benefit, they include:
1. Change the datanode to send a differential report. This saves the namenode
from having to do most of the work in reportDiff(), and avoids the need to
re-link all the unchanged blocks during the "diff" calculation.
2. Keep the linked lists of "to do" work, but modify reportDiff() so that it
can be done under a read lock instead of the write lock. Then only the
processing of the lists needs to be under the write lock. Since the number of
blocks changed is usually much smaller than the number unchanged, this should
improve concurrency.
3. Eliminate the linked lists and just immediately process each changed block
as it is read from the block report. The work on HDFS-1295 indicates that this
might save a large fraction of the total block processing time at scale, due to
the much smaller number of objects created and garbage collected during
processing of hundreds of millions of blocks.
4. As a sidelight to #3, remove linked list use from BlockManager.addBlock().
It currently uses linked lists as an argument to processReportedBlock() even
though addBlock() only processes a single replica on each call. This should be
replaced with a call to immediately process the block instead of enqueuing it.
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