On Tuesday 28 July 2009 16:46:43 Alan Brown wrote:
> On Tue, 28 Jul 2009, Marc Cousin wrote:
> > At peak during the night, we have around 40-50 write streams at the same
> > time, and we are despooling to 3 LTO3 and 3 LTO1.
>
> That's larger than my installation and I haven't got budget for a
> dedicated array controller. We're using software striping on our disks.

I thought so. All I wanted to say was that I think that with proper tuning, 
there is no real need for SSD in this context. I had to use an array for two 
reasons :
- I needed extra capacity : for some jobs, we spool more than one terabyte of 
data before despooling to tape
- Anyway, SDD costed a lot, 2 years ago

SSD wont suck, but they won't be a silver bullet either.

>
> > I think the most important is : don't purchase anything without doing
> > benchmarks first for this special type of activity. That's what we did :
> > we wrote a small program to simulate the expected throughput on the array
> > with several streams, and only bought the array when we were satisfied
> > with its performance, which required a bit of tuning (choose the right
> > filesystem, the right scheduler, tune read ahead...). And I think this
> > time wasn't wasted at all...
>
> You're right, it isn't.
>
> The main criterion is keeping up with the tape drive(s). Right now we are,
> but that will change with a new robot purchase and that's why I'm looking
> ahead.

You'll need to despool in quite small chunks then. Not that it really is a 
problem, it's just that we tend to despool a job when it is finished if 
possible, and for that we need a lot of capacity.

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