Chao Sun created HDFS-14366:
-------------------------------
Summary: Improve HDFS append performance
Key: HDFS-14366
URL: https://issues.apache.org/jira/browse/HDFS-14366
Project: Hadoop HDFS
Issue Type: Improvement
Components: hdfs
Reporter: Chao Sun
Assignee: Chao Sun
In our HDFS cluster we observed that {{append}} operation can take as much as
10X write lock time than other write operations. By collecting flamegraph on
the namenode (see attachment), we found that most of the append call is spent
on {{getNumLiveDataNodes()}}:
{code}
/** @return the number of live datanodes. */
public int getNumLiveDataNodes() {
int numLive = 0;
synchronized (this) {
for(DatanodeDescriptor dn : datanodeMap.values()) {
if (!isDatanodeDead(dn) ) {
numLive++;
}
}
}
return numLive;
}
{code}
this method synchronizes on the {{DatanodeManager}} which is particularly
expensive in large clusters since {{datanodeMap}} is being modified in many
places such as processing DN heartbeats.
For {{append}} operation, {{getNumLiveDataNodes()}} is invoked in
{{isSufficientlyReplicated}}:
{code}
/**
* Check if a block is replicated to at least the minimum replication.
*/
public boolean isSufficientlyReplicated(BlockInfo b) {
// Compare against the lesser of the minReplication and number of live DNs.
final int replication =
Math.min(minReplication, getDatanodeManager().getNumLiveDataNodes());
return countNodes(b).liveReplicas() >= replication;
}
{code}
The way that the {{replication}} is calculated is not very optimal, as it will
call {{getNumLiveDataNodes()}} every time even though usually
{{minReplication}} is much smaller than the latter.
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