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From Uma Mahesh <mahesw...@huawei.com>
Subject Regarding HDFS-693
Date Fri, 01 Jul 2011 04:43:58 GMT
Hi All,
  Anybody facing this issue?
  In our observation this issue came in long run with huge no of blocks in
Data Nodes . every hour Data Nodes are sending their blocks report to the
Name Node. If number of blocks in Data Node are huge (3 Data Nodes with 2GB
RAM, Scribe server is sending logs at 5000records/s , 4 scribe clients ,
block size is 64MB ) then it requires good amount of time to scan all the
blocks. This block scanning causes lot of IO operations. At this time if any
write request comes , then it will take long time for it to get a free io
channel on the Data Node. Because of this during the blcock scan time a Data
Node may not be able to acknowledge the client requests causing timeouts on
the client sockets.
If DN1 send the data to DN2 for replication and at that time DN2 is doing
the block scanning. Since DN2 is busy, it may not be able to send the ack to
DN1 on time. So here timeouts can happen. 
Uma Maheswara Rao .G
Software Engineer
Address: Huawei Industrial Base
Bantian Longgang
Shenzhen 518129, P.R.China
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