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From "Eric Newton (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-8389) HBASE-8354 DDoSes Namenode with lease recovery requests
Date Tue, 23 Apr 2013 15:45:16 GMT

    [ https://issues.apache.org/jira/browse/HBASE-8389?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13639174#comment-13639174
] 

Eric Newton commented on HBASE-8389:
------------------------------------

Hi, accumulo developer here.  I opened HBASE-7878 based on our experience following HBase's
approach to log recovery.

>From our observations:
 * we have never seen the DDoS of the NN while testing on Hadoop 1.0.4 and 2.0.3-alpha
 * accumulo tries to keep the WALs in one HDFS block, too.
 * before checking the return code of recoverLease, we had occasional data loss.
   This was with tests designed to find data loss, while randomly killing servers every few
minutes. These tests would run for hours before detecting data loss.  We were able to attribute
data loss to incomplete log recovery.

accumulo's approach to recovery:
 * it waits a configurable period after the loss of a tablet (region) server lock (default:
10s).
   This gives the server a chance to die, and for zookeeper to propagate agreement about the
lock to the other servers.
 * it calls recoverLease on the WALs until it returns true.
 * due to the discussion in this issue (HBASE-8354), I opened (ACCUMULO-1328) to make the
wait interval between calls to recoverLease configurable: it was hard coded to 1 second.


                
> HBASE-8354 DDoSes Namenode 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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