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From Peter Schüller <>
Subject Re: Read Latency
Date Tue, 11 May 2010 15:15:05 GMT
> 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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