Date: Sun, 03 Apr 2011 16:29:37 -0700 From: "Kevin Oberman" <oberman@es.net> To: Bruce Cran <bruce@cran.org.uk> Cc: freebsd-stable@freebsd.org Subject: Re: Constant rebooting after power loss Message-ID: <20110403232937.E14CA1CC0C@ptavv.es.net> In-Reply-To: Your message of "Sun, 03 Apr 2011 18:20:27 BST." <4D98AC5B.1050606@cran.org.uk>
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> 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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