Messages in this thread | | | From | Ian Stirling <> | Subject | Re: Journaling pointless with today's hard disks? | Date | Tue, 27 Nov 2001 01:23:32 +0000 (GMT) |
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> > On Monday 26 November 2001 15:30, Andre Hedrick wrote: > > On Mon, 26 Nov 2001, Rob Landley wrote: > > > > Just add an off-the-shelf capacitor to your circuit. The firmware > > > already has to detect power failure in order to park the head sanely, so
> > Send me an outline/discription and I will present it during the Dec T13 > > meeting for a proposal number for inclusion into ATA-7. > > What kind of write-up do you want? (How formal?) > > The trick here is limiting the scope of the problem. Your buffer can't be > larger than you can reliably write back on a sudden power failure. (This > should be obvious.) So the obvious answer is to make your writeback cache > SMALL. The problems that go with flushing it are then correspondingly small. <snip> > > Now a cache large enough to hold 2 full tracks could also hold dozens of > individual sectors scattered around the disk, which could take a full second > to write off and power down. This is a "doctor, it hurts when I do this" > question. DON'T DO THAT.
Or, to seek to a journal track, and write the cache to it. Errors are a problem, writing twice may help. This avoids having to block on bad write patterns, for example, if you are writing mixed blocks that go to tracks 1 and 88, you can't start to write blocks that would go to track 44. Performance would rise if it can do the writes in elevator order.
<snip> > That way, the power down problem is strictly limited: > > 1) write out the track you're over > 2) seek to the second track > 3) write that out too > 4) park the head
Or 2) optionally seek to the journal track, and write the journal.
> > What new hardware is involved? > > Add a capacitor. > > Add a power level sensor. (Drives may already have this to know when to park > the head.) Most drives I've taken apart recently seem to have passive means, a spring to move the head to the side, and a magnet to hold it there. <Snip>> > I think that's it. Did I miss anything? Oh yeah, on power fail stop
It needs a power switch to stop back-feeding the computer.
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