documentation of GEOM data structures needed

[Erich Dollansky]

Hi,

On Mon, 11 Nov 2013 15:11:41 +0000
symbolics at gmx.com wrote:

> On Mon, Nov 11, 2013 at 06:32:16PM +0800, Erich Dollansky wrote:
> > 
> > On Mon, 11 Nov 2013 09:18:36 +0000
> > symbolics at gmx.com wrote:
> > 
> > > On Mon, Nov 11, 2013 at 04:24:00PM +0800, Erich Dollansky wrote:
> > > > 
> > > > I would need a documentation of the GEOM data structure. A disk
> > > > got damages in a strange way during the process of backing up
> > > > data. It was partitioned with gpart as all my disks using the
> > > > MBR schema. When creating the backup, the system crashed. The
> > > > disk was not usable anymore. I found out that the MBR was
> > > > overwritten before the backup was started while the beginning
> > > > of the first partition seems to be ok.
> > > > 
> > > > I hope to be able to recover the data saving me one week of
> > > > work.
> > > 
> > > As I understand things, you believe that your filesystems are
> > > okay and you just want to recreate the MBR so you can try to fsck
> > > and mount
> > 
> > this is the basic idea.
> > 
> > > them? How did you set the disc MBR up in the first place? If you
> > > used the automatic scheme in the installer and you know the size
> > > of the
> > 
> > I used the manual way creating 6 partitions.
> 
> Do you know the sizes and order of how they were set up? They're all
> UFS2 or a mixture of other things too?

they are all UFS plus one swap. I have two other which should have the
same partitioning with the exception that the size of the disk differ
by a few sectors. This should only affect the end of the size of the
last partition.
> 
> > > disc, you could reconstruct things that way. What does `gpart
> > > show' look like at the moment?
> > 
> > It does not come that far
> > 
> > gpart list da0
> > gpart: No such geom: da0.
> > 
> > is all I get.
> > 
> > My luck is that I have three disks which are the type but
> > manufactured with some months between. But their sizes differ a
> > bit. I think that I should be able to recover much by just
> > comparing the entries.
> > 
> 
> You can try looking at diskinfo -v da0 to see the numbers.
>
         512             # sectorsize
        500107860480    # mediasize in bytes (466G)
        976773165       # mediasize in sectors
        0               # stripesize
        0               # stripeoffset
        60801           # Cylinders according to firmware.
        255             # Heads according to firmware.
        63              # Sectors according to firmware.
        0000000000006121        # Disk ident.

One other disk shows the same data while the third one shows this:

        512             # sectorsize
        500107862016    # mediasize in bytes (466G)
        976773168       # mediasize in sectors
        4096            # stripesize
        0               # stripeoffset
        15504336        # Cylinders according to firmware.
        1               # Heads according to firmware.
        63              # Sectors according to firmware.
        TF0504YS02ZPBP  # Disk ident.


> I had a little play around making a test disc and mdconfig. If I get
> some more time this evening I'll see if I can write a tool to scan the
> disc and recover the partition data, it'd be a good exercise.
> 
> There are a few programmes in ports/sysutils you should try in the
> meantime. I didn't find them useful in my tests but that might not be
> indicative of anything.
> 
This I will do tomorrow.

Erich