Date: Thu, 3 Mar 2005 16:57:31 -0500 From: Bill Vermillion <bv@wjv.com> To: freebsd-hackers@freebsd.org Subject: Flushing HD cache - was Re: FUD about CGD and GDBE Message-ID: <20050303215731.GC23632@wjv.com> In-Reply-To: <20050303194333.18C6016A4D9@hub.freebsd.org> References: <20050303194333.18C6016A4D9@hub.freebsd.org>
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> ------------------------------ > Message: 18 > Date: Wed, 2 Mar 2005 13:15:49 -0800 (PST) > From: "ALeine" <aleine@austrosearch.net> > Subject: Re: FUD about CGD and GBDE > phk@phk.freebsd.dk wrote: > > I gave up on journalling myself because IMO it complicates > > things a lot and the problem it solves is very very small. > If only hardware manufacturers were to equip hard drives with > a mechanism to ensure atomic writes. A capacitor large enough > to hold enough energy to flush the cache upon detecting the > power supply was cut would be sufficient. They could even use > a battery the status of which could be monitored via S.M.A.R.T., > I don't see how implementing something like that could possibly > make the cost noticably higher. Actually at least some used to do something similar. In some IBM SCSI drives of a few years ago, if there was a power failure, the energy in the rotational momentum of the platters was used as the motor was changed over to a miniature generator that provided enough current to flush the contents of the on-board cache to the platters. A capacitor large enough might be too large considering how small everyone is trying to make thier drives. And the main goal seems to be to make drives cheaper - so until enough people want this feature and are willing to pay for - I don't think we'll see this return. > Recent IBM Thinkpad and Apple PowerBook G4 laptops have sudden > motion sensors which park the disk heads when a sudden motion > is detected in order to prevent damage from a fall and similar, > so this atomic write guarantee mechanism should be trivial for > them to implement and it would save us a lot of work. The first hard drive I ever saw - a genuine Shugart ST-506 - had to be shipped in two boxes with the internal box being suspended by springs from the larger box - as it would only take about .5G before it had problems. At the time it was the only 5.25" hard drive being made. Now drives routinely have 100G shock survivability static and at least 20G when turning. And a human body will not normaly survive a 20G shock. Bill -- Bill Vermillion - bv @ wjv . com
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