linux-ext4 - Re: ext4 filesystem bad extent error review

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Message-ID: <20140103180654.GC4336@thunk.org>
Date:	Fri, 3 Jan 2014 13:06:54 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	"Huang Weller (CM/ESW12-CN)" <Weller.Huang@...bosch.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"Juergens Dirk (CM-AI/ECO2)" <Dirk.Juergens@...bosch.com>
Subject: Re: ext4 filesystem bad extent error review

On Fri, Jan 03, 2014 at 11:54:12AM -0600, Eric Sandeen wrote:
> > 
> > This call chain only happens if the block device is mounted.
> 
> Sure, but I thought that's what they were doing.  Maybe I misread.
> 

I thought this was in relation to doing what they called a "barrier
test", where you are writing to flash device and then drop power, and
then see if the CACHE FLUSH request was actually honored.  (And
whether or not the FTL got corrupted so badly that the device brick's
itself, as does happen for some of the crappier cheap flash out
there.)

But I'm not sure precisely how they implemented their test.  It's
possible it was done with the file system mounted.  My suggestion was
to make sure that the flash was proof against power drops by doing
this using a raw block device, to remove the variable of the file
system.

Given that they've since reported that they can repro the problem
using soft resets, it doesn't sound like the problem is related to
flash devices not handling powe drops correctly --- although given
that I'm still getting reports of people who have had their SD card
get completely bricked after a power drop event, it's unfortunately
not a solved problem by the flash manufacturers yet....  or rather,
the few (many?) bad apples give all low-end flash a bad name.

      	     	     	    	  			 - Ted
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