Re: Constant rebooting after power loss

> Date: Sun, 03 Apr 2011 18:20:27 +0100
> From: Bruce Cran <bruce@cran.org.uk>
> Sender: owner-freebsd-stable@freebsd.org
> 
> On 02/04/2011 04:35, Matthew Dillon wrote:
> 
> >      First, a power loss to the drive will cause the drive's dirty write cache
> >      to be lost, that data will not make it to disk.  Nor do you really want
> >      to turn of write caching on the physical drive.  Well, you CAN turn it
> >      off, but if you do performance will become so bad that there's no point.
> >      So turning off the write caching is really a non-starter.
> 
> Do you know if that's changed at all with NCQ on modern SATA drives? 
> I've seen people commenting that using tags recovers most, if not all, 
> of the performance lost by disabling the write cache.

I may be confused, but I don't think you are interpreting the response
correctly as I don't see nay way that the use of NCQ would begin to
provide the performance of having a write cache. In fact, if I
understand NCQ correctly, it requires that the drive write-cache data so
NCQ can do it's thing.

I believe that point was that properly functioning NCQ (or TCQ) can
assure that the metadata is safely updated so that power loss will never
engender data corruption while enhancing performance. It still will not
save you from losing the data that is in cache and not written, but that
is the extent of the damage.
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman@es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751
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