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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-563) DFS client should try to re-new lease if it gets a lease expiration exception when it adds a block to a file
Date Wed, 27 Sep 2006 20:28:51 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-563?page=comments#action_12438212 ] 
            
Doug Cutting commented on HADOOP-563:
-------------------------------------

If the lease-renewing thread has fallen behind, what confidence do we have that the client
will be able to renew the lease any easier?  Moreover, this approach sounds like it would
place more load on the namenode, which it probably doesn't need if it is being slow to respond
to lease requests.

Perhaps we could instead change the client to wait for the lease-renewing thread to sucessfully
renew things.  But first we should try to better understand the circumstances under which
the lease-renewing thread fails to renew things.  It currently waits for half the lease period,
then starts trying to renew leases once per second until it succeeds.  That sounds like a
fine method to me.  Is the namenode sometimes unresponsive for longer than half lease period?
 If so, then waiting until the renewer succeeds would be an appropriate solution, no?


> DFS client should try to re-new lease if it gets a lease expiration exception when it
adds a block to a file
> ------------------------------------------------------------------------------------------------------------
>
>                 Key: HADOOP-563
>                 URL: http://issues.apache.org/jira/browse/HADOOP-563
>             Project: Hadoop
>          Issue Type: Bug
>            Reporter: Runping Qi
>
> In the current DFS client implementation, there is one thread responsible for renewing
leases. If for whatever reason, that thread runs behind, the lease may get expired. That causes
the client gets a lease expiration exception when writing a block. The consequence of that
is very devastating: the client can no longer write to the file, and all the partial results
up to that point are gone! This is especially costly for some map reduce jobs where a reducer
may take hours or even days to sort the intermediate results before the actual reducing work
can start.
> The problem will be solved if the flush method of  DFS client can renew lease on demand.
That is, it should try to re-new lease  when it catches a lease expiration exception. That
way,  even when under heavy load and the lease renewing thread runs behind, the reducer  task
(or what ever tasks use that client) can preceed.  That will be a huge saving in some cases
(where sorting intermediate results take a long time to finish). We can set a limit on the
number of retries, and may even make it configurable (or changeable at runtime). 
> The namenode can use a different expiration time that is much higher than the current
1 minute lease expiration time for cleaning  up the abandoned unclosed files.
>  

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