Filesystem, RAID Question
rich at math.missouri.edu
Thu Oct 30 16:12:09 PDT 2008
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.
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