Filesystem, RAID Question

[Rich Winkel]

On Wed, Oct 29, 2008 at 07:49:00PM +0000, Matthew Seaman wrote:
> Given that you don't have a BBU, what is the status of write caching
> on the individual hard drives?  You'll have to use 3dm2 or the CLI 
> equivalent to investigate this, as the RAID controller tends to hide 
> that level of information from the OS.  However, this setting is the
> same thing as controlled by the hw.ata.wc sysctl -- and like that 
> it has a major effect on disk IO performance.  Turning write caching 
> off is the safe, conservative thing to do for maximum data security.  

Doesn't hw.ata.wc affect only card-level caching?

It seems likely that the softupdates queuing order might be scrambled
by card-level caching if it juggles pending writes around to minimize
seek times.  If so, it would be disasterous for data integrity in
the event of a power outage.  Disk-level caching might be safe
though ...  Someone needs to ask 3ware whether the card reorders
updates and if so, if there's a setting to keep them in order.

Rich