[CentOS] Building a Back Blaze style POD

Mon May 9 13:59:26 UTC 2011
Lamar Owen <lowen at pari.edu>

On Sunday, May 08, 2011 04:23:23 PM John R Pierce wrote:
>  note that SAS supports N:M multiplexing where 
> any one of the N controller channels can address any of the M 
> devices.... plain SATA only supports 1:M simple expanders

Hmm, that explains how SAS can effectively replace fibre channel at the DAE..... (EMC has gone SAS on their newest midrange storage.....).  For true fibre channel replacement you need dual-attach at the drive; N:M multiplex looks like dual-attach++, at least to my eye.

> And, a significant problem in large drive arrays is mechanical 
> resonance.... you get an array of 24 or whatever disks all being 
> hammered at once in a RAID environment, and the mechanical vibrations 
> can cause interactions which can increase the error rate, this is 
> greatly compounded by a flimsy chassis.

I like the EMC DAE design; it is most definitely not flimsy.  However, I had often wondered about some of the design features of the DAE chassis, and thinking about mechanical resonance makes some things 'click' in my mind that didn't before, things like the thick cast rack ears instead of extending the chassis sheet metal and folding an ear.....

And those EMC DAE's are 15 drive enclosures.  I wonder how much the custom EMC drive firmware impacts mechanical resonance, especially on large RAID groups....