linux-kernel - Re: Linux 2.6.29

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Date:	Mon, 30 Mar 2009 13:05:54 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
cc:	Ric Wheeler <rwheeler@...hat.com>,
	"Andreas T.Auer" <andreas.t.auer_lkml_73537@...us.ath.cx>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Theodore Tso <tytso@....edu>, Mark Lord <lkml@....ca>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	Jeff Garzik <jeff@...zik.org>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rees <drees76@...il.com>, Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29



On Mon, 30 Mar 2009, Rik van Riel wrote:
> 
> Maybe a stupid question, but aren't tracks so small compared to
> the disk head that a physical head crash would take out multiple
> tracks at once?  (the last on I experienced here took out a major
> part of the disk)

Probably. My experiences (not _that_ many drives, but more than one) have 
certainly been that I've never seen a _single_ read error.

> Another case I have seen years ago was me writing data to a disk
> while it was still cold (I brought it home, plugged it in and
> started using it).  Once the drive came up to temperature, it
> could no longer read the tracks it just wrote - maybe the disk
> expanded by more than it is willing to seek around for tracks
> due to thermal correction?   Low level formatting the drive
> made it work perfectly and I kept using it until it was just
> too small to be useful :)

I've had one drive that just stopped spinning. On power-on, it would make 
these pitiful noises trying to get the platters to move, but not actually 
ever work. If I recall correctly, I got the data off it by letting it just 
cool down, then powering up (successfully) and transferring all the data 
I cared about off the disk. And then replacing the disk.

> > And my point is, IT MAKES SENSE to just do the elevator barrier, _without_
> > the drive command. 
> 
> No argument there.  I have seen NCQ starvation on SATA disks,
> with some requests sitting in the drive for seconds, while
> the drive was busy handling hundreds of requests/second
> elsewhere...

I _thought_ we stopped feeding new requests while the flush was active, so 
if you actually do a flush, that should never actually happen. But I 
didn't check.

		Linus
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