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From Steven Rand <stevenjr...@gmail.com>
Subject question about TotalWriteTime in DataNode metrics
Date Mon, 12 Sep 2016 17:49:29 GMT
Hi all,

I was looking at the JMX metrics from one of my DataNodes and noticed that
if I divide BytesWritten by TotalWriteTime, I get a result of about 1000
Mb/sec. However, I can only dd to the disk storing HDFS data on that node
at about 95 Mb/sec.

To try to figure out what's going on, I uploaded some data to HDFS and
checked the values of BytesWritten before and after. BytesWritten changed
by the expected amount (roughly the size of the data uploaded), so those
numbers seem fine. I only have 3 DataNodes and RF=3, so all data is written
to each DataNode during upload.

Therefore it seems that the value of TotalWriteTime must either be too low,
or I'm misunderstanding what it means. My understanding of
BlockReceiver#receivePacket is that TotalWriteTime measures the cumulative
time spent by the DataNode writing to its local disk. Is this correct? If
so, what could explain its value being so low that write throughput seems
to be roughly 10x what the disk is actually capable of?


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