[Tig] Minimum RAID bandwidth for 2K playback?

[Bob Friesenhahn]

On Sun, 20 Jul 2008, Ramona Howard wrote:

>> Note that these "enterprise" SATA drives are slower than higher grade
>> (much more expensive) 15K SAS drives.  The seek time is about 9ms and
>> the transfer rate is just over 100MB/s.  Compare that with seek times
>> of 2ms and a sustained transfer rate of 171MB/s.
>
> * Remember faster drives generate more heat so that plays into an overall
> system design.

There may be a bit more heat, but the biggest factor I notice is the 
noise.  The really fast drives are always much noisier than SATA 
drives designed to fit in someone's desktop.  I bet that Bob's 
collection of 15K SCSI drives is pretty loud.  The latest fast drives 
are always quieter than yesterday's fast drives though.

If you put a "consumer" drive in a RAID array then the RAID array may 
go to sleep for a number of minutes if the drive has difficulty 
reading a sector since consumer drives will do their absolute best to 
recover the bad sector.  The enterprise drive will retry just a few 
times and then return an error since it will assume it is in a RAID 
array and the error will be corrected.

The last thing to keep in mind is that true enterprise drives are 10X 
(and sometimes 100X) less likely to lose data (due to bit error) than 
consumer class drives.  Some enterprise drives have a non-recoverable 
bit error rate of 1x10E16 (vs 1xE14 for consumer SATA drive like the 
Seagate Barracuda 7200.11).  Studies (e.g. a study based on field data 
from Network Appliance) show that many SATA consumer drives tend to 
lose data throughout their entire lifetime whereas the enterprise SAS 
(or true enterprise SATA) drives typically don't lose any data, but if 
they do, then the drive should likely be replaced since it is often on 
a path to failure.

Bob
======================================
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/