linux-kernel - Re: document ext3 requirements

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Message-Id: <200901042149.27655.rob@landley.net>
Date:	Sun, 4 Jan 2009 21:49:26 -0600
From:	Rob Landley <rob@...dley.net>
To:	Theodore Tso <tytso@....edu>
Cc:	Pavel Machek <pavel@...e.cz>, Sitsofe Wheeler <sitsofe@...oo.com>,
	Duane Griffin <duaneg@...da.com>, Valdis.Kletnieks@...edu,
	Martin MOKREJŠ <mmokrejs@...osome.natur.cuni.cz>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, mtk.manpages@...il.com,
	rdunlap@...otime.net, linux-doc@...r.kernel.org
Subject: Re: document ext3 requirements

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.
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