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From Schubert Zhang <zson...@gmail.com>
Subject Re: Read Latency
Date Tue, 11 May 2010 15:40:49 GMT
 "(I originally saw 3-5 ms read latency with a small amount of data and 1
Keyspace/CF)? "

The 3~5ms latency is offered by the Filesystem page cache.
Because your dataset is small, it can be cached totally by Filesystm.


2010/5/11 Peter Schüller <scode@spotify.com>

> > isolated requests, obviously in scale the RAID should perform better... I
> > have not started testing concurrent reads in scale as the single reads
> are
> > too slow to begin with. I am getting 20-30ms response time off of
> internal
>
> Concurrent reads is what you need to do in order to see the benefit of
> a RAID controller with many constituent drives under it.
>
> > drives and 50-70 ms response time through the raid volumes (as reported
> in
> > cfstats). The system is totally idle and all data has been cleanly
>
> That said, a RAID controller imposing several tens of extra
> milliseconds of latency sounds strange; something else or fishy has to
> be going on.
>
> But don't expect sequential reads of non-cached small random-access
> data to be faster with a RAID controller. The benefit of RAID will
> tend to be overall throughput when doing concurrent reads, fast
> fsync() (low latency on durable writes) and the ability to eat bursts
> of write activity in battery backed cache.
>
> --
> / Peter Schuller aka scode
>

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