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From "Aaron T. Myers (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-7097) Allow block reports to be processed during checkpointing on standby name node
Date Thu, 09 Oct 2014 19:45:35 GMT

    [ https://issues.apache.org/jira/browse/HDFS-7097?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14165623#comment-14165623
] 

Aaron T. Myers commented on HDFS-7097:
--------------------------------------

The patch looks pretty good, and in thinking about it a fair bit I think it won't regress
the issue I was trying to address in HDFS-5064, though Kihwal I would appreciate if you could
confirm that as well.

A few small comments:

# Does {{FSNamesystem#rollEditLog}} need to take the nsLock as well? Seems like it might,
given that tailing edits no longer is taking the normal FSNS rw lock.
# Similarly for {{FSNamesystem#(start|end)Checkpoint}}, though that's less obvious to me.
# Seems a little strange to me to be calling this new lock the "nsLock", when that's also
what we've been calling the main FSNS rw lock all this time. I'd suggest renaming this to
the "checkpoint lock" or something, to more clearly distinguish its purpose.
# I think you can now remove some of the other stuff added as part of HDFS-5064, e.g. the
entire {{longReadLock}} I believe was only actually being locked for read during checkpointing.

Thanks a lot for working on this, Kihwal.

> Allow block reports to be processed during checkpointing on standby name node
> -----------------------------------------------------------------------------
>
>                 Key: HDFS-7097
>                 URL: https://issues.apache.org/jira/browse/HDFS-7097
>             Project: Hadoop HDFS
>          Issue Type: Bug
>            Reporter: Kihwal Lee
>            Assignee: Kihwal Lee
>            Priority: Critical
>         Attachments: HDFS-7097.patch, HDFS-7097.patch, HDFS-7097.patch
>
>
> On a reasonably busy HDFS cluster, there are stream of creates, causing data nodes to
generate incremental block reports.  When a standby name node is checkpointing, RPC handler
threads trying to process a full or incremental block report is blocked on the name system's
{{fsLock}}, because the checkpointer acquires the read lock on it.  This can create a serious
problem if the size of name space is big and checkpointing takes a long time.
> All available RPC handlers can be tied up very quickly. If you have 100 handlers, it
only takes 34 file creates.  If a separate service RPC port is not used, HA transition will
have to wait in the call queue for minutes. Even if a separate service RPC port is configured,
hearbeats from datanodes will be blocked. A standby NN  with a big name space can lose all
data nodes after checkpointing.  The rpc calls will also be retransmitted by data nodes many
times, filling up the call queue and potentially causing listen queue overflow.
> Since block reports are not modifying any state that is being saved to fsimage, I propose
letting them through during checkpointing. 



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