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From "Todd Lipcon (Created) (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-3170) Add more useful metrics for write latency
Date Sat, 31 Mar 2012 02:05:25 GMT
Add more useful metrics for write latency
-----------------------------------------

                 Key: HDFS-3170
                 URL: https://issues.apache.org/jira/browse/HDFS-3170
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: data-node
    Affects Versions: 2.0.0
            Reporter: Todd Lipcon


Currently, the only write-latency related metric we expose is the total amount of time taken
by opWriteBlock. This is practically useless, since (a) different blocks may be wildly different
sizes, and (b) if the writer is only generating data slowly, it will make a block write take
longer by no fault of the DN. I would like to propose two new metrics:
1) *flush-to-disk time*: count how long it takes for each call to flush an incoming packet
to disk (including the checksums). In most cases this will be close to 0, as it only flushes
to buffer cache, but if the backing block device enters congested writeback, it can take much
longer, which provides an interesting metric.
2) *round trip to downstream pipeline node*: track the round trip latency for the part of
the pipeline between the local node and its downstream neighbors. When we add a new packet
to the ack queue, save the current timestamp. When we receive an ack, update the metric based
on how long since we sent the original packet. This gives a metric of the total RTT through
the pipeline. If we also include this metric in the ack to upstream, we can subtract the amount
of time due to the later stages in the pipeline and have an accurate count of this particular
link.

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