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From "Varun Sharma (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-8389) HBASE-8354 forces Namenode into loop with lease recovery requests
Date Fri, 26 Apr 2013 19:02:16 GMT

    [ https://issues.apache.org/jira/browse/HBASE-8389?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13643136#comment-13643136

Varun Sharma commented on HBASE-8389:

I can do a small write up that folks can refer to.

One point regarding the low setting though. Its good for fast MTTR requirements such as online
clusters but it does not work well if you pound a small cluster with mapreduce jobs. The write
timeouts start kicking in on datanodes - we saw this on a small cluster. So it has to be taken
with a pinch of salt.

I think 4 seconds might be too tight. Because we have the following sequence -
1) recoverLease called
2) The primary node heartbeats (this can be 3 seconds in the worst case)
3) There are multiple timeouts during recovery at primary datanode:
    a) dfs.socket.timeout kicks in when we suspend the processes using "kill -STOP" - there
is only 1 retry
    b) ipc.client.connect.timeout is the troublemaker - on old hadoop versions it is hardcoded
at 20 seconds. On some versions, the # of retries is hardcoded at 45. This can be trigger
by firewalling a host using iptables to drop all incoming/outgoing TCP packets. Another issue
here is that b/w the timeouts there is a 1 second hardcoded sleep :) - I just fixed it in
HADOOP 9503. If we make sure that all the dfs.socket.timeout and ipc client settings are the
same in hbase-site.xml and hdfs-site.xml. Then, we can

The retry rate should be no faster than 3a and 3b - or lease recoveries will accumulate for
900 seconds in trunk. To get around this problem, we would want to make sure that hbase-site.xml
has the same settings as hdfs-site.xml. And we calculate the recovery interval from those
settings. Otherwise, we can leave a release note saying that this number should be max(dfs.socket.timeout,
ipc.client.connect.max.retries.on.timeouts * ipc.client.connect.timeout, ipc.client.connect.max.retries).

The advantage of having HDFS 4721 is that at some point the data node will be recognized as
stale - maybe a little later than hdfs recovery. Once that happens, recoveries typically occuring
within 2 seconds.
> 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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