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.