ZFS RaidZ2 with 24 drives?

[Bernd Walter]

On Fri, Jan 01, 2010 at 10:56:21AM -0600, Bob Friesenhahn wrote:
> On Fri, 1 Jan 2010, Danny Carroll wrote:
> >
> >You do not have this protection when ZFS has access to the raw devices.
> >Even worse if the devices write cache is turned on.
> 
> This statement does not appear to be true.  ZFS will always request 
> that devices flush their cache.  The only time there is no 
> "protection" is if the device ignores that flush request and the cache 
> is volatile.  Controller battery-backed RAM is useful since the 
> controller can respond to the cache flush request once the data is in 
> battery-backed RAM, thereby dramatically improving write latencies for 
> small writes

Which - if it is true for the controller - can be dangerous.
A battery backed cache is volatile if the system is going down for
a long time.
Or consider the system is going down to relocate the disks to a new
machine, or just to a newer controller?

-- 
B.Walter <bernd at bwct.de> http://www.bwct.de
Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.