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From Sameer Paranjpye <same...@yahoo-inc.com>
Subject Re: dfs datanode heartbeats and getBlockwork requests
Date Wed, 05 Apr 2006 18:53:04 GMT
There's complexity both ways. Not declaring the set of expected data 
nodes introduces a lot of complexity and delay in namenode startup, as 
seen in this thread.

In this respect, it feels like there's a fundamental difference between 
batch systems and storage systems. For a MapReduce JobTracker or Condor 
like system resource usage is transient, a job is executed on a node, it 
completes and exits, the system no longer cares about the node. In a DFS 
nodes store persistent data which mostly stays in place across namenode 
re-starts. Checkpointing the mapping of blocks to nodes would make 
startup much faster. Instead of waiting for datanodes to connect, the 
namenode could poll and find out who was around. Adding nodes does 
require co-ordination with the namenode, but it seems like node addition 
represents a big enough discontinuity for most installations that 
co-ordinating with the namenode is a small price to pay.


Doug Cutting wrote:

> I would rather avoid having to declare the set of expected data nodes if 
> we can avoid it, as I think it introduces a number of complexities.  For 
> example, if you wish to add new data nodes, you cannot simply configure 
> them to point to the name node and start them.  Assuming we add a notion 
> of 'on same rack' or 'on same switch' to dfs, and can ensure that copies 
> of a block are always held on multiple racks/switches, then it's 
> convenient to be able to safely take racks and switches offline and 
> online without coordinating with the namenode.  If a switch fails at 
> startup, and 90% of the expected nodes are not available, we should 
> still start replication, no?  I think a startup replication delay at the 
> namenode handles all of these cases.  If we're worried that the 
> filesystem is unavailable, then we could make the delay smarter.  The 
> namenode could delay some number of minutes or until every block is 
> accounted for, whichever comes first.  And it could refuse/delay client 
> requests until the delay period is over, so that applications don't 
> start up until files are completely available.
> Doug
> Yoram Arnon wrote:
>> Right!
>> The name node, on startup, should know which data nodes are expected 
>> to be
>> there, and not make replication decisions before he knows who's actually
>> there and who's not.
>> A crude way to achieve that is by just waiting for a while, hoping 
>> that all
>> the data nodes connect.
>> A more refined way would be to compare who connected to who is 
>> expected to
>> connect. It enables faster startup when everyone just connects 
>> quickly, and
>> better robustness when some data nodes are slow to connect, or when 
>> the name
>> node is slow to process the barrage of connections.
>> The rule could be "no replications until X% of the expected nodes have
>> connected, AND there are no pending unprocessed connection messages". X
>> should be on the order of 90, perhaps less for very small clusters.
>> Yoram
>> -----Original Message-----
>> From: Hairong Kuang [mailto:hairong@yahoo-inc.com] Sent: Tuesday, 
>> April 04, 2006 5:09 PM
>> To: hadoop-dev@lucene.apache.org
>> Subject: RE: dfs datanode heartbeats and getBlockwork requests
>> I think it is better to implement the start-up delay at the namenode. But
>> the key is that the name node should be able to tell if it is in a steady
>> state or not either at start-up time or at runtime after a network
>> disruption. It should not instruct datanodes to replicate or delete any
>> blocks before it has reached a steady state.
>> Hairong
>> -----Original Message-----
>> From: Doug Cutting [mailto:cutting@apache.org]
>> Sent: Tuesday, April 04, 2006 9:58 AM
>> To: hadoop-dev@lucene.apache.org
>> Subject: Re: dfs datanode heartbeats and getBlockwork requests
>> Eric Baldeschwieler wrote:
>>> If we moved to a scheme where the name node was just given a small 
>>> number of blocks with each heartbeat, there would be no reason to not 
>>> start reporting blocks immediately, would there?
>> There would still be a small storm of un-needed replications on 
>> startup.   Say it takes a minute at startup for all data nodes to 
>> report their
>> complete block lists to the name node.  If heartbeats are every 3 
>> seconds,
>> then all but the last data node to report in would be handed 20 small 
>> lists
>> of blocks to start replicating.  And the switches could be saturated 
>> doing a
>> lot of un-needed transfers, which would slow startup.   Then, for the 
>> next minute after startup, the nodes would be told to delete
>> blocks that are now over-replicated.  We'd like startup to be as fast and
>> painless as possible.  Waiting a bit before checking to see if blocks are
>> over- or under-replicated seems a good way.
>> Doug

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