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From Robert Coli <>
Subject Re: Tracking down read latency
Date Thu, 03 Feb 2011 05:50:39 GMT
On Wed, Feb 2, 2011 at 9:35 PM, David Dabbs <> wrote:
> We’re encountering some high read latency issues.

What is reporting high read latency?

> We're more read than write, though there doesn't seem to be many pending
> reads.
> I have seen active/pending row-read at three or four, though.

In general if you were I/O bound on reads (the most common
pathological case) you would see much higher row-read stage pending.

> [ sane looking tpstats ]

Your tpstats does not look like a node which is struggling.

> avg-cpu:  %user   %nice %system %iowait  %steal   %idle
>            1.00    0.00    0.25    1.32    0.00   97.43

Your system also seems to not be breaking a sweat.

> We're running on a beefy 64-bit Nehalem, so mmap should be
> available/possible.
> I need to check with our Cassandra lead when he's available as to why we're
> not using mmap or auto.

Probably because of :

> Heap size is 16gb.

16gb out of how much total?

Do your GC logs seem to indicate reasonable GC performance? Do all
nodes generally have a complete view of the ring and all nodes
generally seem to be up?


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