NameNode seems unstable on a cluster with little space left
-----------------------------------------------------------
Key: HADOOP-3810
URL: https://issues.apache.org/jira/browse/HADOOP-3810
Project: Hadoop Core
Issue Type: Bug
Components: dfs
Affects Versions: 0.17.1
Reporter: Raghu Angadi
Assignee: Raghu Angadi
NameNode seems not very responsive and unstable when the cluster has very
little space left. The clients timeout. The main problem is that it is not
clear the user what is going on. Once I have more details about a NameNode that
was in this state, I will fill in here.
If there is not enough space left on a cluster, it is ok for clients to receive
something like "DiskOutOfSpace" exception.
Right now it looks like NameNode tries too hard find a node with any space left
and ends up beeing slow to respond to clients. If the CPU take by
chooseTarger() is the main reason, there two fixes possible :
# chooseTarget() iterates and takes quite a bit of CPU for allocating
datanodes. Usually this not much of a problem. It takes even more cpu when it
needs to search multiple racks for a datanode. We could probably reduce some
CPU for these allocations. The benefit should be measurable.
# Also, once NameNode can not find any datanode that has space on a rack, it
could mark the rack as "full" and skip searching the rack for next one minute
or so. This flag gets cleared after a minute or if any new node is added to the
rack.
#* Of course, this might not be optimal w.r.t disk space usage.. but only for a
short duration. Once a cluster is mostly full, the user does expect errors.
#* On the flip side, this fix does not require extremely CPU optimized version
of chooseTarget().
#* I think it is reasonable for NameNode to throw DiskOutOfSpace exception,
even though it could have found space if it searched extensively.
--
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.
