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Re: A little RAID experiment

To: xfs@xxxxxxxxxxx
Subject: Re: A little RAID experiment
From: Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>
Date: Tue, 17 Jul 2012 21:18:18 -0500
In-reply-to: <20120717052621.GB23387@dastard>
References: <CAAxjCEzh3+doupD=LmgqSbCeYWzn9Ru-vE4T8tOJmoud+28FDQ@xxxxxxxxxxxxxx> <CAAxjCEzEiXv5Kna9zxZ-ePbhNg6nfRinkU=PCuyX3QHesq5qcg@xxxxxxxxxxxxxx> <5004875D.1020305@xxxxxxxxxxxxxxxxx> <CAAxjCEw-NJzZmX3Q5CJ+aZ_Q7Yo39pMU=-hiXk0ghTMq7q3PWA@xxxxxxxxxxxxxx> <5004C243.6040404@xxxxxxxxxxxxxxxxx> <20120717052621.GB23387@dastard>
Reply-to: stan@xxxxxxxxxxxxxxxxx
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On 7/17/2012 12:26 AM, Dave Chinner wrote:
...
> I bet it's single threaded, which means it is:

The data given seems to strongly suggest a single thread.

> Which means throughput is limited by IO latency, not bandwidth.
> If it takes 10us to do the write(2), issue and process the IO
> completion, and it takes 10us for the hardware to do the IO, you're
> limited to 50,000 IOPS, or 200MB/s. Given that the best being seen
> is around 35MB/s, you're looking at around 10,000 IOPS of 100us
> round trip time. At 5MB/s, it's 1200 IOPS or around 800us round
> trip.
> 
> That's why you get different performance from the different raid
> controllers - some process cache hits a lot faster than others.
...
> IOWs, welcome to Understanding RAID Controller Caching Behaviours
> 101 :)

It would be somewhat interesting to see Stefan's latency and throughput
numbers for 4/8/16 threads.  Maybe the sysbench "--num-threads=" option
is the ticket.  The docs state this is for testing scheduler
performance, and it's not clear whether this actually does threaded IO.
 If not, time for a new IO benchmark.

-- 
Stan

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