EWF corrupted question / concern

  • Thread starter Thread starter Guest
  • Start date Start date
G

Guest

Hi,
Before asking some EWF questions I was reviewing all posting regarding
this matter and I saw that Kevin W on 6/23/2005 posted "What if file is
corrupted in the overlay?"
This post contains exactly the same questions I was going to ask.
At this time Matt Kellner(MS) provided good answers and redirected to Kevin
to force-shutdown escenarios and disk overlays info document ( very
interesting ) located at:
http://blogs.msdn.com/embedded/archive/2005/06/15/429225.aspx

So, After reading this link which basically says that even a EWF protected
partition can be corrupted after a power fail my question is:

What is the advantage of EWF partitions if they are under the same risk of
corruption when a power fail occurs?. If the MBR gets corrupted the OS will
not be able to boot.

I have been reading a lot about this because I have the intent of migrating
my system to use a Disk EWF partition; but apparently there is something I'm
missing.

if someone has a response to this please let me know.

Thanks for your help
GL
 
I had similar concerns, in terms of a disk overlay, the only real advantage I
can see is that you can call “ewfmgr c: -restore†to remove the corruption,
assuming the OS will boot. In the end I've gone for a three partition
approach, one protected by a RAM overlay, another partition for the page file
and event logs, then a third partition which we use for our applications and
data. The reason for moving the page file and event logs to another disk was
because they both seem to write straight through the ewf layer. With this
system we can maintain state on the disks, yet the OS should be rock solid
(indeed it's been on soak test for about 10,000 active power cycles without
an issue). If the application drive becomes corrupt we have a proprietary
client that can clear down the disk and update the terminal back to its
correct state. This seems to be the most reliable and flexible approach. That
said I’m not an expert and others may well say what I’ve done if daft.
 
Hi

I just read the articel from the embedded. As I see it, its only a problem
on systems with overlays on harddisk. When using af flash medie (Compact
flash) the power loos problem isn't there. Am I complete of here ??

If I have made a commit to my CF card and the power loos occure when
flushing I could damage files etc. I know...

/Thomas
 
Hi GL. To answer your question, there really isn't any inherent "advantage"
to a disk overlay in an unpredictable-power situation like the one I
described in my blog post. EWF will provide an extra layer of protection
against corruption and compromise, but as I pointed out in my blog post,
there are always risks whenever doing any sort of write operation to the
hard drive. EWF Disk Overlays are not designed to protect against every
conceivable scenario in which corruption could occur.

That said, they do offer a couple of handy features, such as multiple
overlay "levels" that can be discarded one at a time to restore the system
to a previous state between boots. It is also possible to discard the
overlay from the F8 boot menu if an overlay file becomes corrupted and
causes the system to fail to boot.

Generally speaking, if your system does not need to persist data between
boots, I recommend you use a RAM or RAM Reg overlay. This overlay will
prevent any data from being written to your hard drive at all, so risk of
damage is absolutely minimal. But if you need to persist data between
boots, there are a couple of ways to tackle it.

As Rob White pointed out, you can partition your hard drive (or CF device)
into multiple partitions. Your first partition would contain your OS boot
files and any non-changing application files and data, and it would be
protected by an EWF RAM overlay. Then your subsequent partition(s) would
contain the data you'd want to persist between boots. This solution would
still minimize your risk of write failure because the system would only
actually write to the hard drive whenever one of those persistent files is
updated, but not when it does something with the OS (such as update the
registry). Non-critical system info would be lost when you power the system
off, but the system would still be able to boot, and your application could
pick up on the current persistent data. =)

I hope this helps.

--
Matt Kellner ([email protected])
STE, Windows Embedded Group

This posting is provided "AS IS" with no warranties, and confers no rights.
===============================
 
Matt,
Thanks for your comments.
According to my understanding the very worse scenario would be a Disk EWF
partion that gets corrupted in case of power loss because the system was
writing to the overlay or to an extra partition and the HDD heads swings back
to their parked position ( as you mention in your document ) ?. The other
corrupted areas could be validated in some way. Am I right?, I if so this
cold not happen if a CF is used?. Or is there any known way a CF could be
corrupted also? or reliability issues?

This question is because I would have a 40G HDD ( enough space for me ) and
adding a CF will increase my cost, but if CF has some issues also I don't see
a justification for the extra cost.

Rob White comments make me feel better; I will try also that aproach

thanks Rob.

Thanks Again.
GL
 
I forgot to include an extra Question.
Is there any issue if I disable Event Logs to avoid disk access?. Could
this make the OS to crash some way?

Thanks
GL
 
I don't what happens if you disable them, but you can just move them to write
to the hard drive that you mentioned. It's just a registry setting. That way
you don't need to worry about the OS being strange. That said, I can't
imagine disabling them would cause the OS to become that wobbly.
 
Back
Top