Greg,
Again, you raise some interesting issues. I wonder how likely the
catastrophic failures you describe are, versus how likely it is that
things fail in a way where ccd actually helps you. I was hoping someone
else would comment on that, but that doesn't seem to have happened so
far.
> So one thing that's still missing is a big, bold line at the top
> that says:
>
> CCD Mirroring will eventually eat your data and you shouldn't use it!!
It's missing, because I am not at all convinced that claim is true.
The way I see it: when you use CCD mirroring, your data is written to
multiple disks, rather than just one. In some situations, this won't
help you (all your disks die in a fire; you delete your own files; ...)
In some situations, this will help you (one of your disks fails, but you
still have correct data on others). In some situations, it is not as
good as other techniques (the cases you describe). It may or may not
still be better than no mirroring in these cases (for example, in the
case where one file gets corrupted, you may still have everything else
intact).
I definitely think that stating that CCD mirroring _will_ eat your data
is FUD; short of bugs, CCD doesn't cause you to lose data; at worst, it
may not preserve data which other methods would have preserved.
> To promote the use of CCD Mirroring without noting the above major
> problems is a disservice to the novice who is likely not aware of
> the above failure modes.
You are right that it would be deceptive to advertize CCD mirroring as a
silver bullet. It would be a lie to say CCD mirroring is the best
mirroring method. However, my HOWTO does neither of these. It clearly
mentions that mirroring is no silver bullet (and that goes for _any_
kind of mirroring), and that RAID is superior to CCD. The HOWTO might
actually not emphasize these points enough; I'll have a look at it
sometime and make changes if I deem them necessary.
> To me, until the above have satisfactory
> answers, the only thing the CCD Mirroring HOWTO (and the ccd(4)/
> ccdconfig(8) man-pages!) should recommend is:
>
> Don't use CCD Mirroring -- at best, it provides a false sense of
> security. At worst, it will eat your data. If you need mirroring
> functionality, use RAIDframe.
Again, you're making bold claims. I would like if someone else could
comment on them. Does CCD mirroring really provide only a false sense of
security? Will it really eat your data? Or is it just that it's not as
good as RAIDframe, but still a valuable improvement over not using any
mirroring at all?
> Really. RAIDframe works, and it doesn't suffer from the serious
> problems noted above.
Agreed. However, RAIDframe requires compiling a custom kernel. Now. And
when you next upgrade your system. And the next time. Until it gets
included in the shipped kernel. CCD is easy to set up (once you figure
out the steps) and I think it provides some protection against harddisk
failures.
Again, thanks for your comments.
Bob
PS. If anybody on the list is annoyed by this discussion continuing
despite people already having pointed out that my HOWTO is considered
harmful, please tell me so. Until that happens, I'm assuming people are
Ok with me discussing things here and I'll keep responding to messages
people send me.
---
A man should practice what he preaches, but a man should also preach
what he practices.
-- Confucius
