Re: document ext3 requirements

[Robert Hancock]

Rob Landley wrote:
> On Sunday 04 January 2009 17:30:52 Theodore Tso wrote:
> > On Sun, Jan 04, 2009 at 11:40:52PM +0100, Pavel Machek wrote:
> > > Not neccessarily.
> > >
> > > If I have a bit of precious data and lot of junk on the card, I want
> > > to copy out the precious data before the card dies. Reading the whole
> > > media may just take too long.
> > >
> > > That's probably very true for rotating harddrives after headcrash...
> > For a small amount data, maybe; but the number of seeks is often far
> > more destructive than the amount of time the disk is spinning.  And in
> > practice, what generally happens is the user starts looking around to
> > make sure there wasn't anything else on the disk worth saving, and now
> > data is getting copied off based on human reaction time.  So that's
> > why I normally advise users that doing a full image copy of the disk
> > is much better than, say, "cp -r /home/luser /backup", or cd'ing
> > around a filesystem hierarchy and trying to save files one by one.
>
> That would be true if the disk hardware wasn't doing a gazillion retries to 
> read a bad sector internally (taking 5 seconds to come back and report 
> failure), and then the darn scsi layer added another gazillion retries on top 
> of that, and the two multiply together to make it so slow that that when you 
> leave the thing copying the disk overnight it's STILL not done 24 hours later.  
> Going in and cherry picking individual files looks kind of appealing in that 
> situation.
>
> Rob
>
> P.S. Yeah, I had a laptop hard drive crash a month or so back.  I remember 
> when it was still possible to buy storage devices that didn't get arbitrarily 
> routed through the SCSI layer.  I miss those days.  I found the patch to route 
> ramdisks through the scsi layer amusing, though.

SCSI layer doesn't do any retries itself. Block layer does.

Even with zero software retries however, if there are a ton of bad 
sectors it can still take ages for them to all fail reading one at a 
time just from the disk's retries..
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