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From Brian Bockelman <bbock...@cse.unl.edu>
Subject Re: DataBlockScanner scan period
Date Thu, 14 Oct 2010 00:18:25 GMT
Hi Thanh,

That is correct.  Last time I read the code, Hadoop scheduled the block verifications randomly
throughout the period in order to avoid periodic effects (i.e., high load every N minutes).

Brian

On Oct 13, 2010, at 7:14 PM, Thanh Do wrote:

> Brian,
> 
> When you say *attempt* to complete and *entire* node scan,
> you mean for example, if a node has 100 block files, it will
> try to verify all 100 block every 3 weeks?
> That is in average, a block is scanned every (3 weeks / 100 time interval)?
> 
> Thanks
> Thanh
> 
> 
> On Wed, Oct 13, 2010 at 7:07 PM, Brian Bockelman <bbockelm@cse.unl.edu>wrote:
> 
>> Hi Thanh,
>> 
>> The scan period is the period that hadoop *attempts* to complete an entire
>> node scan.  That is, if it's set to 3 weeks, HDFS will try to scan each
>> block once every 3 weeks.
>> 
>> Obviously, depending on the bandwidth you have made available to the
>> scanning thread, you can specify impossibly small periods.
>> 
>> Brian
>> 
>> On Oct 13, 2010, at 7:01 PM, Thanh Do wrote:
>> 
>>> Hi again,
>>> 
>>> Could any body explain to me about the scanning period
>>> policy of DataBlockScanner? That is who often it wake up
>>> and scan a block file.
>>> When looking at the code, I found
>>> 
>>> static final long DEFAULT_SCAN_PERIOD_HOURS = 21*24L; // three weeks
>>> 
>>> 
>>> but definitely it does not wake up and pick a random block
>>> to verify every three weeks, right?
>>> 
>>> Thanks a lot,
>>> Thanh
>> 
>> 


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