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From Steve Loughran <ste...@apache.org>
Subject Re: dfs.data.dir
Date Thu, 29 Apr 2010 09:28:37 GMT
Scott Carey wrote:
> On Apr 26, 2010, at 9:45 AM, Steve Loughran wrote:
> 
>> Allen Wittenauer wrote:
>>> On Apr 22, 2010, at 5:41 AM, Steve Loughran wrote:
>>>> that brings up a couple of issues I've been thinking about now that workers
can go to 6+ HDDs/node
>>>>
>>>> * a way to measure the distribution across disks, rather than just nodes.
DfsClient doesn't provide enough info here yet.
>>> What should probably happen is that instead of throwing you to the file browser,
clicking on a host from the live nodes page should probably put you on a "stats about this
node" page.
>> I don't want to do any of this by hand. I want machine readable content 
>> something can aggregate over time.
>>
>>>> * a way to triger some rebalancing on a single node, to say "position stuff
more fairly". You don't need to worry about network traffic, just local disk load and CPU
time, so it should be simpler.
>>>
>>> Yup.  Working with 8 drives per node, it is interesting to see how unbalanced
the data gets after a while.  [Luckily, we have MR tmp space segregated off so I'm sure it
would be a lot worse if we didn't!]
>>>
>>> Someone should file a jira. :)
>> Especially if someone else offers to fix it.
>>
> 
> Should be trivial to at least make the new block allocation choose which device to allocate
the block on with a weighted roulette algorithm instead of round-robin.
> 

I'd go for another plugin point with the default impl being round-robin, 
let people come up with other strategies, and add a way from DfsClient 
to measure distribution.

Other strategies could use specific knowledge about the disks -are they 
raided, can you hot swap, is the disk playing up, etc, etc, feature 
creep that I encourage people to explore.

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