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MapReduce, mail # user - Re: High IO Usage in Datanodes due to Replication


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Re: High IO Usage in Datanodes due to Replication
Harsh J 2013-05-01, 08:55
Hi,

Neither block reports nor block scanning should affect general DN I/O,
although the former may affect DN liveliness in older versions, if
they lack HDFS-2379 in them. Brahma is partially right in having
mentioned the block reports, hence.

Your solution, if the # of blocks per DN is too high (counts available
on Live Nodes page in NN UI), say > 1m or so blocks, is to simply
raise the DN heap by another GB to fix issues immediately, and then
start working on merging small files together for more efficient
processing and reducing overall block count to lower memory pressure
at the DNs.

On Wed, May 1, 2013 at 12:02 PM, selva <[EMAIL PROTECTED]> wrote:
> Thanks a lot Harsh. Your input is really valuable for me.
>
> As you mentioned above, we have overload of many small files in our cluster.
>
> Also when i load data huge data to hive tables, It throws an exception like
> "replicated to to 0 nodes instead of 1". When i google it i found one of the
> reason matches my case  "Data Node is Busy with block report and block
> scanning" @ http://bit.ly/ZToyNi
>
> Is increasing the Block scanning and scanning all inefficient small files
> will fix my problem ?
>
> Thanks
> Selva
>
>
> On Wed, May 1, 2013 at 11:37 AM, Harsh J <[EMAIL PROTECTED]> wrote:
>>
>> The block scanner is a simple, independent operation of the DN that
>> runs periodically and does work in small phases, to ensure that no
>> blocks exist that aren't matching their checksums (its an automatic
>> data validator) - such that it may report corrupt/rotting blocks and
>> keep the cluster healthy.
>>
>> Its runtime shouldn't cause any issues, unless your DN has a lot of
>> blocks (more than normal due to overload of small, inefficient files)
>> but too little heap size to perform retention plus block scanning.
>>
>> > 1. Is data node will not allow to write the data during
>> > DataBlockScanning process ?
>>
>> No such thing. As I said, its independent and mostly lock free. Writes
>> or reads are not hampered.
>>
>> > 2. Is data node will come normal only when "Not yet verified" come to
>> > zero in data node blockScannerReport ?
>>
>> Yes, but note that this runs over and over again (once every 3 weeks
>> IIRC).
>>
>> On Wed, May 1, 2013 at 11:33 AM, selva <[EMAIL PROTECTED]> wrote:
>> > Thanks Harsh & Manoj for the inputs.
>> >
>> > Now i found that the data node is busy with block scanning. I have TBs
>> > data
>> > attached with each data node. So its taking days to complete the data
>> > block
>> > scanning. I have two questions.
>> >
>> > 1. Is data node will not allow to write the data during
>> > DataBlockScanning
>> > process ?
>> >
>> > 2. Is data node will come normal only when "Not yet verified" come to
>> > zero
>> > in data node blockScannerReport ?
>> >
>> > # Data node logs
>> >
>> > 2013-05-01 05:53:50,639 INFO
>> > org.apache.hadoop.hdfs.server.datanode.DataBlockScanner: Verification
>> > succeeded for blk_-7605405041820244736_20626608
>> > 2013-05-01 05:53:50,664 INFO
>> > org.apache.hadoop.hdfs.server.datanode.DataBlockScanner: Verification
>> > succeeded for blk_-1425088964531225881_20391711
>> > 2013-05-01 05:53:50,692 INFO
>> > org.apache.hadoop.hdfs.server.datanode.DataBlockScanner: Verification
>> > succeeded for blk_2259194263704433881_10277076
>> > 2013-05-01 05:53:50,740 INFO
>> > org.apache.hadoop.hdfs.server.datanode.DataBlockScanner: Verification
>> > succeeded for blk_2653195657740262633_18315696
>> > 2013-05-01 05:53:50,818 INFO
>> > org.apache.hadoop.hdfs.server.datanode.DataBlockScanner: Verification
>> > succeeded for blk_-5124560783595402637_20821252
>> > 2013-05-01 05:53:50,866 INFO
>> > org.apache.hadoop.hdfs.server.datanode.DataBlockScanner: Verification
>> > succeeded for blk_6596021414426970798_19649117
>> > 2013-05-01 05:53:50,931 INFO
>> > org.apache.hadoop.hdfs.server.datanode.DataBlockScanner: Verification
>> > succeeded for blk_7026400040099637841_20741138
>> > 2013-05-01 05:53:50,992 INFO
>> > org.apache.hadoop.hdfs.server.datanode.DataBlockScanner: Verification

Harsh J