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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HADOOP-1306) DFS Scalability: Reduce the number of getAdditionalBlock RPCs on the namenode
Date Mon, 14 May 2007 17:38:16 GMT

     [ https://issues.apache.org/jira/browse/HADOOP-1306?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

dhruba borthakur updated HADOOP-1306:
-------------------------------------

    Attachment: fineGrainLocks2.patch

Here is a first version of changing the locking so that getAdditionalBlock and addStoredBlock
occur without any global locks. I have seen that randownwriter and dfsio that used to fail
on a 1000 node cluster now runs successfully with this patch.

1. NetworkTopology has reader/writer locks. This map hardly changes but is used very frequently.
Now, multiple open() calls can proceed in parallel.

2. The pending blocks and pending files are put into a new class called pendingCreates.java.
This helps locking them together.

3. The BlocksMap is protected by a reader/writer lock.

4. In the common case (when the file is still in pendingCreates), addStordBlock() does not
acquire the global fsnamesystem lock.

5. The datanodeMap was already using a lock object associated with it to protect modifications
to it. Make sure that this check is done in all places where the datanodeMap is modified.

6. The Host2NodesMap has its own read/write lock. This will be merged in with the datanodeMap
when we go to a much finer locking model in future.

This patch is for code review purposes only. Some additional locking is needed for processReport
(still to be done) but I would like some comments on the changes I have made.

I would have liked a more-finer grain locking model that allows all filesystem-methods to
be highly-concurrent. But that approach was deemed too-complex for the short term. I am putting
out this patch to get feedback on whether this medium-term approach is acceptable.  





> DFS Scalability: Reduce the number of getAdditionalBlock RPCs on the namenode
> -----------------------------------------------------------------------------
>
>                 Key: HADOOP-1306
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1306
>             Project: Hadoop
>          Issue Type: Improvement
>          Components: dfs
>            Reporter: dhruba borthakur
>         Attachments: fineGrainLocks2.patch
>
>
> One of the most-frequently-invoked RPCs in the namenode is the addBlock() RPC. The DFSClient
uses this RPC to allocate one more block for a file that it is currently operating upon. The
scalability of the namenode will improve if we can decrease the number of addBlock() RPCs.
One idea that we want to discuss here is to make addBlock() return more than one block. This
proposal came out of a discussion I had with Ben Reed. 
> Let's say that addBlock() returns n blocks for the file. The namenode already tracks
these blocks using the pendingCreates data structure. The client guarantees that these n blocks
will be used in order. The client also guarantees that if it cannot use a block (dues to whatever
reason), it will inform the namenode using the abandonBlock() RPC. These RPCs are already
supported.
> Another possible optimization : since the namenode has to allocate n blocks for a file,
should it use the same set of datanodes for this set of blocks? My proposal is that if n is
a small number (e.g. 3), it is prudent to allocate the same set of datanodes to host all replicas
for this set of blocks. This will reduce the CPU spent in chooseTargets().

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