[cAos-devel] Cinch partition_scan (was Re: System Imager)

[Greg Kurtzer]

On Feb 28, 2007, at 12:28 AM, Martyn wrote:

> Greg Kurtzer wrote:
>> Dang, I wrote that code eons ago!
> I know that feeling!
>> I think what happened was that I was originally also grabbing the  
>> partition type which you can't get from /proc/partitions. Then I  
>> removed that logic for something else and didn't rewrite the  
>> function to use /proc/partitions.
> That's not really what it looks like, here's what I think it's doing :
>
> scan_partitions seems to read /proc/partitions, then to check that  
> the partition is really accessible you run fdisk on the *partition*  
> and if it outputs an error, assume it's a valid partition.

Yea, I think you read the code correctly. As I mentioned, eons. hehe

> Unfortunately, there appear to be a few situations (partition used  
> to be an extended, is now a primary is common) where fdisk will  
> output a partition table /from inside/ a partition.

Thats weird. Who would do such a dastardly deed?

>  I know I had a hard time getting this info through to Michael on  
> irc (too many opportunities for misunderstanding), so I'll repeat  
> and rephrase : scan_partitions looks at /proc/partitions and sees  
> for example hda1-4, so you run fdisk /dev/hda1 and check for error  
> to see if it's a valid partition; rinse and repeat.  If for example  
> hda4 used to be an extended partition, there's no error output, so  
> cinch ignores it.  That seems to be the same for some XP NTFS  
> partitions too as fdisk somehow interprets some data at the start  
> of the partition (I'd assume a backup of the partition table within  
> the fs, but not certain).

Michael probably didn't get what you were saying because it seems,  
uhm,... rather wacked out. Well, I guess now we know who would do  
such a thing. ;)

Extended partitions shouldn't show up here, so this is expected  
output. partitions_display will contain a list of all partitions that  
can be formatted and mounted/installed to.

Now, why a NTFS partition wouldn't error out on a fdisk -l is wacky.  
Being that I can't test on any systems directly, I am open for  
suggestions.

> As I say, the workround is to obliterate the start of all the valid  
> partitions, then cinch will get an error from them, but I don't  
> think that's a "solution" by any means.

Well, that definitely fixes the *Windows* being installed issue,  
don't it? hehe

> Hopefully I've explained well enough, but ask for more  
> clarification and I'll try and explain differently.  If you think  
> there's a possibility of the kernel not catching up with the  
> partitions or something, I'd suggest using dd to verify we can read  
> from the partition instead of fdisk (If that's why it's doing it!).

I think I am getting it, but just unable to reproduce locally so  
kinda hard to debug and test other workarounds.

As I mentioned, I am open to suggestions. :)

Thanks,
Greg

--
Greg Kurtzer

    I believe the world would be a better place if people didn't  
believe in their beliefs. -- gmk