On Wed, Dec 14, 2005 at 06:33:00PM +0100, Markus Wernig wrote:
> Joachim Schipper wrote:
>
> > There was a lengthy thread about ccd mirroring here. Search the
> > archives, and check whether it's worth the risk of ccd 'eating your
> > data' first. (If not, go with RAID-1.)
>
> Hi
>
> Yes, I followed the thread, but to my understanding it was not
> conclusive that ccd would be such a great risk - there were at least two
> diverging opinions.
>
> I tried mounting the device with softdep, which yielded 7 MB/s write
> throughput - better, but not overwhelming.
>
> That said, I then tried to simulate a crash (hit power putton on idle
> box, no obvious write activity going on on the device) - and the
> filesystem came back with unrecoverable errors. I will try to
> investigate this further and determine if it was just bad fs luck or the
> underlying ccd device.
>
> Anyway, I wanted to avoid building a custom kernel, so I might just
> rsync the two disks from cron.
I agree that the ccd mirroring debate ended with at least one
participant on each side retaining his first opinion. However,
considering one was pretty much a newbie and the other the RAIDFrame
maintainer, I know who *I* believe.
Never mind that I'd pretty much figured out that ccd mirroring has the
problems he mentioned on my own, some time earlier when setting up RAID,
and decided not to get burnt.
FWIW, RAIDFrame and ccd mirroring are not very good if you want maximal
speed, especially write speed. They are also complex. Ccd mirroring
is very vulnerable to crashes, and while RAID will deal with them more
elegantly, it *will* require quite a bit of time to rewrite parity.
For reference, here's what I do...
Important documents are in a subversion repository on a RAID-1
/var and /var/mail are on a RAID-1
/, /usr and /home are copied nightly (via the altroot mechanism,
expanded to altroot/-usr/-home via /etc/daily.local - see afterboot(8),
/etc/daily)
Additionally, all of the system but /tmp is backed up nightly, to tape.
Joachim
