linux-kernel - Re: document ext3 requirements

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Message-ID: <49618D2C.3090004@shaw.ca>
Date:	Sun, 04 Jan 2009 22:31:40 -0600
From:	Robert Hancock <hancockr@...w.ca>
To:	Rob Landley <rob@...dley.net>
CC:	Theodore Tso <tytso@....edu>, 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

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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