Date: Tue, 3 Mar 1998 22:35:07 +0100 (MET) From: Wilko Bulte <wilko@yedi.iaf.nl> To: mike@smith.net.au (Mike Smith) Cc: shimon@simon-shapiro.org, sbabkin@dcn.att.com, tlambert@primenet.com, jdn@acp.qiv.com, blkirk@float.eli.net, hackers@FreeBSD.ORG, grog@lemis.com Subject: Re: SCSI Bus redundancy... Message-ID: <199803032135.WAA03917@yedi.iaf.nl> In-Reply-To: <199803032123.NAA17949@dingo.cdrom.com> from Mike Smith at "Mar 3, 98 01:23:24 pm"
next in thread | previous in thread | raw e-mail | index | archive | help
As Mike Smith wrote... > > > > On 03-Mar-98 Wilko Bulte wrote: > > ... > > > > > This is called the 'write hole' in the literature. The trick is to > > > use battery backed cache not only for RAID5 (write)performance > > > reasons, but also to keep the data until date AND parity have safely > > > landed on the disks. > > > > I have seen an interesting solution some time ago; Instead of battery, the > > spindle motor (on the disk) was used to generate the power needed to flush > > the caches. then the motor leads will be clamped, and the spidle shut down > > quickly (normal procedure nowdays). This was done on a 14" spindle that > > had a bit more inertia than todays' disks. But the circuitry consumed more > > power too. I think Digital RL02 disks did that, maybe even RK05 (2.5 Mb on 14".....) > This is common practice on most modern disks; it dates from the days of > autoparking stepper-motor units in that market. > > You don't get a guarantee that everything that you wrote to the cache > on the disk will be flushed, just that the disk won't write half a > block and trail off. Some do flush the cache, some will complete the > current set of blocks, and some will process as much of the cache as > they have power for. It seems to depend on the studliness of the > firmware authors, and TBH given that most of this understanding comes > from experimental rather than authoratative sources you are welcome to > take your own interpretation. The oldies did not have any cache, they used the rotational energy to autopark their heads (and slam a mechanical lock in place). Wilko _ ______________________________________________________________________ | / o / / _ Bulte email: wilko @ yedi.iaf.nl http://www.tcja.nl/~wilko |/|/ / / /( (_) Arnhem, The Netherlands - Do, or do not. There is no 'try' --------------- Support your local daemons: run [Free,Net,Open]BSD Unix -- To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?199803032135.WAA03917>