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From "Nicolas Liochon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-8389) HBASE-8354 forces Namenode into loop with lease recovery requests
Date Fri, 26 Apr 2013 08:20:18 GMT

    [ https://issues.apache.org/jira/browse/HBASE-8389?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13642669#comment-13642669

Nicolas Liochon commented on HBASE-8389:

Varun, I +1 Stack: the timeout setting you mentionned are quite impressive!
Thanks a lot for all this work.

Here is my understanding, please correct me where I'm wrong.

In don't think that single / multiple block is an issue, even if it's better to have single
block (increased parallelism).

HBase has a dataloss risk: we need to wait for the end of recoverFileLease before reading.
 => Either by polling the NN and calling recoverFileLease multiple times
 => Either calling isFileClosed (HDFS-4525) (and polling as well) where it's available.

I'm not sure that we can poll every second recoverFileLease. When I try I have the same logs
as Eric: "java.io.IOException: The recovery id 2494 does not match current recovery id 2495
for block", and the state of the namenode seems strange. 

In critical scenarios, the recoverFileLease won't happen at all. The probability is greatly
decreased by HDFS-4721, but it's not zero.

In critical scenarios, the recoverFileLease will start, but will be stuck in bad datanodes.
The probability is greatly decreased by HDFS-4721 and HDFS-4754, but it's not zero. Here,
we need to limit the number of retry in HDFS to one, whatever the global setting, to be on
the safe side (no hdfs jira for this).

I see a possible common implementation (trunk / hbase 0.94)
 - if HDFS-4754, calls markAsStale to be sure this datanode won't be used.
 - call recoverFileLease a first time
 - if HDFS-4525 is available, call isFileClosed every second to detect that the recovery is
 - every 60s, call again recoverFileLease (either isFileClosed is missing, either we went
into one of the bad scenario above). 

This would mean: no dataloss and a MTTR of:
 - less than a minute if we have stale mode + HDFS-4721 + HDFS-4754 + HDFS-4525 + no retry
in HDFS recoverLease or Varun's settings.
 - around 12 minutes if we have none of the above. But that's what we have already without
the stale mode imho.
 - in the middle if we have a subset of the above patches and config.

As HDFS-4721 seems validated by the HDFS dev team, I think that my only question is: can we
poll very frequently recoverFileLease if we don't have isFileClosed?

As a side node, tests more or less similar to yours with HBase trunk and HDFS branch-2 trunk
(without your settings but with a hack to skip the deadnodes) brings similar results.

> HBASE-8354 forces Namenode into loop with lease recovery requests
> -----------------------------------------------------------------
>                 Key: HBASE-8389
>                 URL: https://issues.apache.org/jira/browse/HBASE-8389
>             Project: HBase
>          Issue Type: Bug
>            Reporter: Varun Sharma
>            Assignee: Varun Sharma
>            Priority: Critical
>             Fix For: 0.94.8
>         Attachments: 8389-0.94.txt, 8389-0.94-v2.txt, 8389-0.94-v3.txt, 8389-0.94-v4.txt,
8389-0.94-v5.txt, 8389-0.94-v6.txt, 8389-trunk-v1.txt, 8389-trunk-v2.patch, 8389-trunk-v2.txt,
8389-trunk-v3.txt, nn1.log, nn.log, sample.patch
> We ran hbase 0.94.3 patched with 8354 and observed too many outstanding lease recoveries
because of the short retry interval of 1 second between lease recoveries.
> The namenode gets into the following loop:
> 1) Receives lease recovery request and initiates recovery choosing a primary datanode
every second
> 2) A lease recovery is successful and the namenode tries to commit the block under recovery
as finalized - this takes < 10 seconds in our environment since we run with tight HDFS
socket timeouts.
> 3) At step 2), there is a more recent recovery enqueued because of the aggressive retries.
This causes the committed block to get preempted and we enter a vicious cycle
> So we do,  <initiate_recovery> --> <commit_block> --> <commit_preempted_by_another_recovery>
> This loop is paused after 300 seconds which is the "hbase.lease.recovery.timeout". Hence
the MTTR we are observing is 5 minutes which is terrible. Our ZK session timeout is 30 seconds
and HDFS stale node detection timeout is 20 seconds.
> Note that before the patch, we do not call recoverLease so aggressively - also it seems
that the HDFS namenode is pretty dumb in that it keeps initiating new recoveries for every
call. Before the patch, we call recoverLease, assume that the block was recovered, try to
get the file, it has zero length since its under recovery, we fail the task and retry until
we get a non zero length. So things just work.
> Fixes:
> 1) Expecting recovery to occur within 1 second is too aggressive. We need to have a more
generous timeout. The timeout needs to be configurable since typically, the recovery takes
as much time as the DFS timeouts. The primary datanode doing the recovery tries to reconcile
the blocks and hits the timeouts when it tries to contact the dead node. So the recovery is
as fast as the HDFS timeouts.
> 2) We have another issue I report in HDFS 4721. The Namenode chooses the stale datanode
to perform the recovery (since its still alive). Hence the first recovery request is bound
to fail. So if we want a tight MTTR, we either need something like HDFS 4721 or we need something
like this
>   recoverLease(...)
>   sleep(1000)
>   recoverLease(...)
>   sleep(configuredTimeout)
>   recoverLease(...)
>   sleep(configuredTimeout)
> Where configuredTimeout should be large enough to let the recovery happen but the first
timeout is short so that we get past the moot recovery in step #1.

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