Date: Mon, 24 Jan 2011 14:56:48 +0000 From: Karl Pielorz <kpielorz_lst@tdx.co.uk> To: Bruce Cran <bruce@cran.org.uk> Cc: freebsd-fs@freebsd.org Subject: Re: Write cache, is write cache, is write cache? Message-ID: <DF3B02CFFAFEC6344F08045B@HexaDeca64.dmpriest.net.uk> In-Reply-To: <20110123221651.000037f2@unknown> References: <1ABA88EDF84B6472579216FE@Octa64> <20110122111045.GA59117@icarus.home.lan> <D8077758CFED5EF1CB2AB613@Octa64> <20110122172714.00002274@unknown> <1032CC037935BA2DC2E60A4F@Octa64> <20110123221651.000037f2@unknown>
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--On 23 January 2011 22:16 +0000 Bruce Cran <bruce@cran.org.uk> wrote: > You might also find http://brad.livejournal.com/2116715.html > interesting. Ok, the utility provided via the Blog reports zero errors when run from both UFS, and ZFS with the BIOS set to 'Enabled drive write cache: No'. Flip the BIOS to 'Yes' - for UFS, things get messy (as you'd kind of expect) - the file system gets trashed (enough that autoboot / background checking fails), and you can't complete the test because the test file doesn't exist after reboot (side effect of softupdates I'd guess). For ZFS the test still completes - with 'zero errors'. Checking via 'camcontrol' - the BIOS option 'Enabled drive write cache' just seems to affect the 'Write cache enable' features on the drive, i.e. " test# camcontrol identify ada0 | grep -E "(Feature)|(cache)" Feature Support Enabled Value Vendor write cache yes no flush cache yes yes " And with it set to 'Yes' in the BIOS: " test# camcontrol identify ada0 | grep -E "(Feature)|(cache)" Feature Support Enabled Value Vendor write cache yes yes flush cache yes yes " So in a very round-the-houses way, that's all it appears to change... Guess it's nice to have the option. I'd also guess whilst enabling it for ZFS is good, having it enabled for UFS is slightly dangerous (and that by enabling it machine wide you may trash the UFS file systems on that machine if it suffers a catastrophic power outage). -Karl
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