On Monday 27 July 2009 15:21:17 Alan Brown wrote:
> On Fri, 24 Jul 2009, Marc Cousin wrote:
> > All you really need is to be able to read and write big streams at the
> > same time. So the real problem is to help your disk scheduler to be able
> > to read while having a lot of data in the write cache.
>
> How many streams are you handling simultaneously.
>

Sorry, it took me a while to get that information ...


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.

Load is sustained around 250MB/s for reads from spool and 300MB/s for writes 
to spool.

Of course, the disk array costs more than 2 64GB X25-E ... But at the same 
time, 128 GB would be really small for us.

Anyway, the purpose of my message wasn't to tell "don't buy SSD". You'll have 
better throughput with 1 SSD than with 1 SCSI 15k drive. But you could 
purchase a good controller and normal (SCSI 10k or maybe SATA) drives, and 
have a similar or better throughput with more that a few minutes worth of 
spool (128GB is roughly 5 minutes here :) )

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...

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