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From Andrzej Bialecki ...@getopt.org>
Subject Datanode / namenode UUIDs (Re: "lost" NDFS blocks following network reorg)
Date Wed, 29 Mar 2006 08:27:55 GMT
Stefan Groschupf wrote:
> Hi hadoop developers,


My comments below. Generally speaking, I think you are right - datanodes 
should be initialized with a UUID, created once and persisted across IP 
changes and hostname changes, and this UUID should be used to identify 
datanodes/namenodes. I think the "format" command should also be 
implemented for datanodes, to create their UUID when starting for the 
first time - and later on this UUID should be retrieved from the local 
file somewhere in the data dir.

> The local name of the data node is  machineName + ":" + tmpPort.  So 
> it can change if the port is blocked or the machine name change.
> May we should create the datanode only once and write it to the data 
> folder to be able read it later on.(?)
> This local name is used to send block reports to the name node. 
> FSNamesystem#processReport(Block newReport[], UTF8 dataNodeLocalName) 
> process this report.
> In the first line of this method the DatanodeInfo is loaded by the 
> dataNode's localName. The datanode already is in this map since a 
> heart beat is send before a block report.
> So:
>   DatanodeInfo node = (DatanodeInfo) datanodeMap.get(name);  // no 
> problem but just a 'empty' container:
> ...
>   Block oldReport[] = node.getBlocks(); // will return null since no 
> Blocks are yet associated with this node.
> Since oldReport is null all code is skipped until line 901. But this 
> only adds the blocks to the node container.

Umm.. I don't follow. The lines 901-905 will add these blocks from the 
newReport, because newPos == 0.

> In line 924 begins a section of code that collects all obsolete 
> blocks. First of all I wondering why we iterate throw all blocks here, 
> this could be expansice and it would be enough to iterate over all 
> blocks that are reported by this datanode, isn't it?
> If a block is still valid is tested by FSDirectory#isValidBlock that 
> checks if the block is in activeBlocks.
> The problem I see now is that the only method that adds Blocks t 
> activeBlocks is unprotectedAddFile(UTF8 name, Block blocks[]). But 
> here also the name node local name that may changed is involved.
> This method is also used to load the state of stopped or crashed name 
> node.
> So in case you stop the dfs, change host names a set of blocks will be 
> marked as obsolete and deleted.

I'm not 100% sure if this part is correct, but it makes me nervous, too, 
to involve such ephemeric things as IP/hostname in handling data that 
persists across IP/hostname changes...

Best regards,
Andrzej Bialecki     <><
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