[OpenIndiana-discuss] System disk corruption

[Richard Elling]

On Feb 20, 2012, at 6:38 AM, Robin Axelsson wrote:
> 
> Maybe the iostat "behavior" depends on the controller it monitors. Some controllers such as the AMD SB950 in my case may not be as transparent with errors as the LSI 1068e operating in IT mode.
> 
> Still, I find this to be too much of a coincidence. It is evident that ZFS is not very good to use without disk redundancy.

Eh? Other file systems will blissfully deliver corrupted data. Silent data 
corruption is a much worse fate!

> I'll try to add a mirror to the system pools as soon as possible. It would be great if there were some kind of software that could be set up to generate .par2 files (with x% data redundancy) on-the-fly to protect files on hard drives without disk redundancy (RAID=0).

Not needed. ZFS has a copies parameter where you can set the number of
redundant copies on a per-dataset basis. For example, you can set copies=2
for important data, and copies=1 (the default) for data stored on other media
(eg .iso files)

OTOH, par2 is a completely different architecture that is designed for transferring
files reliably. par2 is not well suited for direct access to data.

> I couldn't recover the image file with cp but I learned in the process that it is possible with dd. 'dd if=infile of=outfile conv=noerror,sync' could do it.

Correct, cp will exit on a failed read.

> Then I discovered ddrescue which did *exactly* what I expected cp to do. I just entered:
> 
> # ddrescue /path/to/corrupted/file /path/to/recovered/file /path/to/logfile.log

Good idea.

> all paths were even in the same vdev. In the process the vdev became 'DEGRADED' even though no additional corruption occurred. So I did a scrub afterwards and 'zfs clear':ed the error afterwards. I did an fmadm repair to tell fma about it. Perhaps I should fmadm reset zfs-diagnosis and zfs-retire as well.

Once you've recovered the data, why are you so interested in eliminating the history of
the corruption?

> Neither par2 nor ddrescue are included with OpenIndiana, I downloaded and installed them manually from the opencsw.org repository. I would strongly recommend to have such tools included with OI.

par2 seems to have little traction. ddrescue can be useful, but is only applicable in rare cases.
 -- richard

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