Date: Fri, 18 Jan 2013 10:18:56 -0800 (PST) From: Scott Long <scott4long@yahoo.com> To: Wojciech Puchar <wojtek@wojtek.tensor.gdynia.pl> Cc: "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org>, Dieter BSD <dieterbsd@gmail.com>, "gibbs@FreeBSD.org" <gibbs@freebsd.org>, "scottl@FreeBSD.org" <scottl@freebsd.org>, "mjacob@FreeBSD.org" <mjacob@freebsd.org> Subject: Re: IBM blade server abysmal disk write performances Message-ID: <1358533136.41693.YahooMailNeo@web120302.mail.ne1.yahoo.com> In-Reply-To: <alpine.BSF.2.00.1301181909250.12638@wojtek.tensor.gdynia.pl> References: <CAA3ZYrCgMmGi3EHKEuXb=qWPjC2zSMYcfgZ6nh-ipqQ7dAeVdA@mail.gmail.com> <1358529778.71931.YahooMailNeo@web120304.mail.ne1.yahoo.com> <alpine.BSF.2.00.1301181909250.12638@wojtek.tensor.gdynia.pl>
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----- Original Message ----- > From: Wojciech Puchar <wojtek@wojtek.tensor.gdynia.pl> > To: Scott Long <scott4long@yahoo.com> > Cc: Dieter BSD <dieterbsd@gmail.com>; "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org>; "gibbs@FreeBSD.org" <gibbs@freebsd.org>; "scottl@FreeBSD.org" <scottl@freebsd.org>; "mjacob@FreeBSD.org" <mjacob@freebsd.org> > Sent: Friday, January 18, 2013 11:10 AM > Subject: Re: IBM blade server abysmal disk write performances > >> >> The default value, -1, instructs the driver to leave the STA drives at > their configuration default. Often times this means that the MPT BIOS will turn > off the write cache on every system boot sequence. IT DOES THIS FOR A GOOD > REASON! An enabled write cache is counter to data reliability. Yes, it helps > make benchmarks look really good, and it's acceptable if your data can be > safely thrown away (for example, you're just caching from a slower source, > and the cache can be rebuilt if it gets corrupted). And yes, Linux has many > tricks to make this benchmark look really good. The tricks range from buffering > the raw device to having 'dd' recognize the requested task and > short-circuit the process of going to /dev/null or pulling from /dev/zero. I > can't tell you how bogus these tests are and how completely irrelevant they > are in predicting actual workload performance. But, I'm not going to stop > anyone from trying, so give the above tunable a try >> and let me know how it works. >> > If computer have UPS then write caching is fine. even if FreeBSD crash, > disk would write data > I suspect that I'm encountering situations right now at netflix where this advice is not true. I have drives that are seeing intermittent errors, then being forced into reset after a timeout, and then coming back up with filesystem problems. It's only a suspicion at this point, not a confirmed case. Scott
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