how to boot or access problem file system

[Roland Smith]

On Thu, Jul 30, 2009 at 03:20:55PM -0400, PJ wrote:
> Roland Smith wrote:
> > On Thu, Jul 30, 2009 at 01:40:58PM -0400, PJ wrote:
> >   
> >> What can be done to access a file system that seems to have the boot
> >> sector screwed up?

I forgot to mention that your boot sector is fine. If it were screwed
up, you wouldn't get to the boot prompt.

Since the boot code cannot locate your kernel, there are several things
that could have gone wrong. See below.

<snip>
> >> The /usr files should be ok but how to access?   
> >
> > Use fsck_ffs to try and repair the filesystem.
> >   
> how can I use it if I can't boot or access the file system?

Use a livefs cd or use the Fixit option in the main menu of sysinstall
on an install disk. That should get you a shell where you can run
fsck_ffs on your disk partitions.

If you have booted from CD, list the disk devices with e.g. 'ls
/dev/ad*'. If you have SCSI drives, use 'da' instead of 'ad'.
What does that command list? On my machine, I'll get
something like this:

/dev/ad4         /dev/ad4s1d      /dev/ad6         /dev/ad6s1d
/dev/ad4s1       /dev/ad4s1e      /dev/ad6s1       /dev/ad6s1e
/dev/ad4s1a      /dev/ad4s1f      /dev/ad6s1a      /dev/ad6s1f
/dev/ad4s1b      /dev/ad4s1g      /dev/ad6s1b      /dev/ad6s1g
/dev/ad4s1c      /dev/ad4s1g.eli  /dev/ad6s1c      /dev/ad6s1g.eli

If you only see e.g. /dev/ad4 and /dev/ad6, your slice table has been
overwritten (with fdisk) and your data is lost. If you see /dev/ad4s1
but not /dev/ad4s1a-g, the BSD partitions have been removed and your
data is lost as well.

Since there is only one slice on both ad4 and ad6 (otherwise you'd see
/dev/ad4s2x) The next step is to examine the disk labels:

bsdlabel /dev/ad4s1
# /dev/ad4s1:
8 partitions:
#        size   offset    fstype   [fsize bsize bps/cpg]
  a:  1024000       16    4.2BSD     2048 16384 64008 
  b: 16777216  1024016      swap                    
  c: 976768002        0    unused        0     0         # "raw" part, don't edit
  d:  4194304 17801232    4.2BSD     2048 16384 28528 
  e: 104857600 21995536    4.2BSD     2048 16384 28528 
  f: 41943040 126853136    4.2BSD     2048 16384 28528 
  g: 807971826 168796176    4.2BSD     2048 16384     0

This tells us that the a, d, e, f and g partition are carrying a BSD
filesystem, and should be checked with fsck_ffs.

Try these steps and report back what you find.

> >> I don't have a problem with irrecoverable files, I would just finally
> >> understand how things work and what can be done on FBSD.
> >
> > Make regular backups. Especially before big upgrades.
> >   
> Maybe the real problem is that the manual is too screwed up (why are
> there so many problems being brought up on the mailing lists? we can't
> all be that stupid.)

It is a mailing list for questions. Ipso facto you'll see questions and
problems on this list. People who are not having problems will not be
posting very much. :-) 

As to the handbook, this is by necessity written by people who are
knowledgeable on the subject they write on. Unfortunately this sometimes
lead to really basic steps/assumptions being skipped because they are
self-evident for the writer. If you gain enough knowledge about a
subject it becomes really hard to write for people new to the system
because you've internalized a lot of stuff by then.

If you have specific questions about parts of the handbook, ask.


Roland
-- 
R.F.Smith                                   http://www.xs4all.nl/~rsmith/
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