"Cannot find file system superblock" error - how to recover?

Fri Dec 5 02:54:39 PST 2003

On Thu, 4 Dec 2003 21:07:20 -0800 (PST)
"Scott I. Remick" <scott at sremick.net> probably wrote:

> 
> --- Sergey 'DoubleF' Zaharchenko <doublef at tele-kom.ru> wrote:
> > I've got a (probably bad) idea. If you say that the partition was
> > mounted as /data, then you could do a
> > 
> > # hd /dev/ad6s1 |grep /data
> > 
> > It should come up soon (the superblock should be close to the beginning
> > of the drive, right?). This way you can at least figure out where your
> > superblock lies (rounding the address of `/data' to 8K). Considering the
> > above discussion, you can calculate the *correct* address of the `e'
> > partition by subtracting 8K or 64K or 256K. See if it matches the one in
> > the disklabel.
> > 
> > Of course, this is all possible only if your superblock isn't screwed
> > enough to NOT contain `/data'.
> 
> Been running about a minute so far... nada. So I guess your assumption is
> correct: the 1st superblock is destroyed (as fsck suggested when it barfed).

I wonder what did destroy it. Of course, system crashes can do wonders,
but...

> > Just a minute. Are you sure that the filesystem was newfs'd with the
> > default parameters? If it were for me to newfs it, I would probably
> > choose larger block&fragment sizes, as I would probably be storing large
> > files. The superblock copy positions depend on the block/frag size. If
> > you specify parameters different from those used for actually newfs'ing
> > it the very first time, newfs -N will give you *incorrect* copy
> > addresses!
> 
> Well, specifying custom block/frag sizes is a bit out of my customization
> forte at the moment, and certainly at the time this drive went in. I'm 99%
> positive I used sysinstall to set it up. I remember some quirks about the
> sysinstall method, and also deciding that the by-hand method was
> unnecessarily complicated for my needs. 
> 
> This has taught me that, should I ever choose to do that, that writing down
> these custom values is CRITICAL.
> 
> Is there any way to positively identify a superblock location (say, using hd
> | grep ) using known information? Just a random thought.

In fact, there should be a way, because a valid superblock copy has a
correct checksum. Perhaps I'll hack up a program to do that taking
information from the /usr/src/sys/ufs/... There's also a magic number
for a superblock, mentioned in fs.h (in 4.8 it's 0x011954). So, for me,
grepping gives

$ hd </dev/ad2s2 | grep "54 19 01 00"
00002550  01 00 00 00 00 00 00 00  00 00 00 00 54 19 01 00  |............T...|
00004550  01 00 00 00 00 00 00 00  00 00 00 00 54 19 01 00  |............T...|
001e9150  11 00 07 00 54 19 01 00  a4 b2 3a c0 00 00 00 00  |....T...╓╡:ю....|
00a97570  8b 48 60 81 b9 5c 05 00  00 54 19 01 00 75 13 8b  |H`╧\...T...u.|
00a98900  05 00 00 54 19 01 00 74  0b 68 54 5f 2d c0 e8 91  |...T...t.hT_-юХ|
00efb150  11 00 07 00 54 19 01 00  a4 b2 3a c0 00 00 00 00  |....T...╓╡:ю....|
013b3f50  00 54 19 01 00 75 13 8b  51 30 81 fa 00 00 01 00  |.T...u.Q0З....|
013b52d0  00 00 8b 70 0c 81 be 5c  05 00 00 54 19 01 00 74  |..p.╬\...T...t|
01705760  8b 48 60 81 b9 5c 05 00  00 54 19 01 00 75 13 8b  |H`╧\...T...u.|
01706af0  05 00 00 54 19 01 00 74  0b 68 34 89 2a c0 e8 91  |...T...t.h4*юХ|
01831f50  00 54 19 01 00 75 13 8b  51 30 81 fa 00 00 01 00  |.T...u.Q0З....|
018332d0  00 00 8b 70 0c 81 be 5c  05 00 00 54 19 01 00 74  |..p.╬\...T...t|
.
.
.

Only the first two are real --- the superblock and the first copy (I'm
on UFS1). The rest are false positives. You might want to try using 

$ hd </dev/ad2s2 | grep "54 19 01 00  |"

to filter out most of them. Have fun.

If the disk is that screwed, you might also want to try out tct
(/usr/ports/sysutils/tct). I'm not sure, but it might be helpful
later(?)

> Although I'm treating this as a learning experience, I also REALLY REALLY
> don't want to loose all that data. I do appreciate the help you've been
> giving me. Thanks again. I'm choosing to remain optimistic. I used to
> salvage lots of data from DOS/Windows partitions (still do) so learning the
> tricks of the trade in my new OS of choice is important to me.
> 
> (PS: already pricing out external USB hard drive enclosures for making
> backups of this drive in the future)

A good idea would also be to print out the hd of the superblock contents
on a sheet of paper when you find it and put it in a cool dry place:)

> _______________________________________________
> freebsd-questions at freebsd.org mailing list
> http://lists.freebsd.org/mailman/listinfo/freebsd-questions
> To unsubscribe, send any mail to "freebsd-questions-unsubscribe at freebsd.org"
> 

-- 
DoubleF
New Year's Eve is the time of year when a man most feels his age, and
his wife most often reminds him to act it.
		-- Webster's Unafraid Dictionary
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 187 bytes
Desc: not available
Url : http://lists.freebsd.org/pipermail/freebsd-questions/attachments/20031205/5564fa5c/attachment.bin