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From Peter Schuller <>
Subject Re: flush_largest_memtables_at messages in 7.4
Date Wed, 13 Apr 2011 21:27:59 GMT
> I still don't understand. You would expect read latency to increase
> drastically when it's fully saturated and lot of READ drop messages also,
> correct? I don't see that in cfstats or system.log which I don't really
> understand why.

No. With a fixed concurrency there is only so many outstanding
requests at any given moment. Unless the concurrency is sufficiently
huge to exhaust queue sizes that's not what you see. It's just the way
it works.

An analogous situation is this: You have N threads all doing random
reads from your disk. If each thread is responsible for exactly one
outstanding I/O operation at a time, you will never have more than N
outstanding requests to the I/O device in total. On average then, when
a request starts it will have N - 1 requests in "front" of it waiting
to be processes, so the latency will increase linearly with N. Double
the concurrency, double the latency.

Something like Cassandra/postgresql/etc behaves similarly. If you're
running a benchmark at a fixed concurrency of N, you won't have more
than N total requests outstanding at any given moment.

/ Peter Schuller

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