Ext3 users - Sep 2002 - EXT3 Corruption Question

Greetings Everyone,

	A group in my office is designing/integrating what I would loosly label as an
"embedded system".  In that, it performs a dedicated task and is not a
general purpose workstation or server.   It operates in a harsh environment,
particularly a questionable power environment and they are extremely concerned
about inadvertent shutdown and recovery.  They were totally unaware of
journalling filesystems in general so I filled them in on ext3.  I am quite an
ext3 novice myself ( boy do I LOVE it on my laptop! ) so when they come to me
with questions, I hedge my answers, and I don't like to do that.

	Here is their inquery in a nutshell.  1)  The filesystem is on a battery backed
up solid state device.  2)  they do not care about the time necessary for
recovery and avoiding corruption is paramount.   3)  OS is a non-RTOS  4)  The
power just went out, you have 20 milliseconds.....    How can corruption be
avoided in this case?  CAN it be avoided?   If not, under what type of
circumstances would the filesystem become corrupt (so we can mitigate these
circumstances)?

Thanks very much.

	

Regards,

Gary Huntress
Code 4113
Naval Undersea Warfare Center
Newport, RI  02841
1-800-669-6892 x28990

On Sep 05, 2002  13:36 -0400, Huntress Gary B NPRI
wrote:
>	A group in my office is designing/integrating what I would loosly
> label as an "embedded system".	In that, it performs a dedicated
task
> and is not a general purpose workstation or server.   It operates in
> a harsh environment, particularly a questionable power environment and
> they are extremely concerned about inadvertent shutdown and recovery.
> They were totally unaware of journalling filesystems in general so I
> filled them in on ext3.  I am quite an ext3 novice myself ( boy do I
> LOVE it on my laptop! ) so when they come to me with questions, I hedge
> my answers, and I don't like to do that.
If it is writing to a flash device, I'd suggest using JFFS, as it is
better suited to "limited write" devices like flash.

Otherwise, just use "data=journal" and you will avoid most types of
corruption, depending on what the application is doing.
>	Here is their inquery in a nutshell.  1)  The filesystem is on
> a battery backed up solid state device.  2)  they do not care about the
> time necessary for recovery and avoiding corruption is paramount.
> 3) OS > is a non-RTOS  4)  The power just went out, you have 20
> milliseconds.....
> How can corruption be avoided in this case?  CAN it be avoided?   If not,
> under what type of circumstances would the filesystem become corrupt
> (so we can mitigate these circumstances)?
I suppose it depends on what you mean by corruption.  metadata
journaling (the default) will prevent filesystem structure corruption.
ordered data mode (default) will prevent you from seeing "corrupt"
files on disk (corrput = old data that was not overwritten when the
file size grew).

Cheers, Andreas
--
Andreas Dilger
http://www-mddsp.enel.ucalgary.ca/People/adilger/
http://sourceforge.net/projects/ext2resize/