Hi!

quoted

I can reproduce data loss with ext3 on flashcard in about 40
seconds. I'd not call that "odd event". It would be nice to handle
that, but that is hard. So ... can we at least get that documented
please?
  

Part of documenting best practices is to put down very specific things  
that do/don't work. What I worry about is producing too much detail to  
be of use for real end users.

Well, I was trying to write for kernel audience. Someone can turn that
into nice end-user manual.

I have to admit that I have not paid enough attention to this specifics  
of your ext3 + flash card issue - is it the ftl stuff doing out of order  
IO's? 

The problem is that flash cards destroy whole erase block on unplug,
and ext3 can't cope with that.

quoted

_All_ flash cards (MMC, USB, SD) had the problems. You don't need to
get clear grasp on trends. Those cards just don't meet ext3
expectations, and if you pull them, you get data loss.
  

Pull them even after an unmount, or pull them hot?

Pull them hot.

[Some people try -osync to avoid data loss on flash cards... that will
not do the trick. Flashcard will still kill the eraseblock.]

quoted

quoted

Nothing is perfect. It is still a trade off between storage 
utilization  (how much storage we give users for say 5 2TB drives), 
performance and  costs (throw away any disks over 2 years old?).
    

"Nothing is perfect"?! That's design decision/problem in raid5/ext3. I
believe that should be at least documented. (And understand why ZFS is
interesting thing).
  

Your statement is overly broad - ext3 on a commercial RAID array that  
does RAID5 or RAID6, etc has no issues that I know of.

If your commercial RAID array is battery backed, maybe. But I was
talking Linux MD here.

quoted

And I still use my zaurus with crappy DRAM.

I would not trust raid5 array with my data, for multiple
reasons. The fact that degraded raid5 breaks ext3 assumptions should
really be documented.

Again, you say RAID5 without enough specifics.  Are you pointing just at  
MD RAID5 on S-ATA? Hardware RAID cards? A specific commercial RAID5 
vendor?

Degraded MD RAID5 on anything, including SATA, and including
hypothetical "perfect disk".

quoted

The papers show failures in "once a year" range. I have "twice a
minute" failure scenario with flashdisks.

Not sure how often "degraded raid5 breaks ext3 atomicity" would bite,
but I bet it would be on "once a day" scale.

We should document those.

Documentation is fine with sufficient, hard data....

Degraded MD RAID5 does not work by design; whole stripe will be
damaged on powerfail or reset or kernel bug, and ext3 can not cope
with that kind of damage. [I don't see why statistics should be
neccessary for that; the same way we don't need statistics to see that
ext2 needs fsck after powerfail.]
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html