Date: Thu, 30 Oct 2008 18:12:07 -0500 From: Rich Winkel <rich@math.missouri.edu> To: freebsd-questions@freebsd.org Subject: Re: Filesystem, RAID Question Message-ID: <20081030231207.GA56260@pencil.math.missouri.edu> In-Reply-To: <4908BE2C.7010505@infracaninophile.co.uk> References: <9f3798c00810291118i1c80cb8cw8d4995eabe6a4f8f@mail.gmail.com> <4908BE2C.7010505@infracaninophile.co.uk>
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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
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