[lkml]   [2001]   [Nov]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: Journaling pointless with today's hard disks?
> 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.
> 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.

> 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.
> 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.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2005-03-22 13:13    [W:0.158 / U:1.460 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site