detached a mounted ufs filesystem

Ian Smith smithi at
Sun Jun 13 20:15:05 UTC 2010

In freebsd-questions Digest, Vol 314, Issue 13, Message: 22
On Sat, 12 Jun 2010 22:38:15 -0400 "Xihong Yin" <xyin at> wrote:

 > 'fdisk /dev/da0' output is
 > ******* Working on device /dev/da0 *******
 > parameters extracted from in-core disklabel are:
 > cylinders=14593 heads=255 sectors/track=63 (16065 blks/cyl)
 > Figures below won't work with BIOS for partitions not in cyl 1
 > parameters to be used for BIOS calculations are:
 > cylinders=14593 heads=255 sectors/track=63 (16065 blks/cyl)
 > Media sector size is 512
 > Warning: BIOS sector numbering starts with sector 1
 > Information from DOS bootblock is:
 > The data for partition 1 is:
 > sysid 165 (0xa5),(FreeBSD/NetBSD/386BSD)
 >  start 63, size 234436482 (114470 Meg), flag 80 (active)
 >  beg: cyl 0/ head 1/ sector 1;
 >  end: cyl 1023/ head 254/ sector 63

Ok, the DOS partition table looks likely intact; it says you have one 
slice ad0s1 occupying the entire disk _after_ the first 63 sectors.

Checking .. 14593 * 255 * 63 = 234436545 sectors, less
DOS partition 1 (FreeBSD s1) = 234436482 sectors equals 63 sectors,
being all of cylinder 0, head 0; slices start on cylinder boundaries.

 > The data for partition 2 is:
 > The data for partition 3 is:
 > The data for partition 4 is:
 > I tried 'fsck_ufs /dev/da0', it says
 > ** /dev/da0
 > Cannot find file system superblock
 > ioctl (GCINFO): Inappropriate ioctl for device
 > fsck_ufs: /dev/da0: can't read disk label

Yes, as it should.  You haven't lost your MBR & partition table, but it 
appears you may have lost the bsdlabel at the beginning of slice 1; this 
is likely why FreeBSD only finds /dev/da0 and (perhaps) /dev/da0a ..

In the absence of a label on slice 1 (listing FreeBSD partitions c, the 
whole slice, and one or more of a, b, d - h) the system seems to assume 
this is a 'dangerously dedicated' disk, with no slices, covering the 
whole disk - but still with no label.  I think the system is probably
assuming wrongly here, from a missing or damaged label.

Polytropon's advice to dd the whole drive to somewhere is excellent if 
you have 114GB spare somewhere - but if you don't, at least dd the first 
126 or so sectors to a file in case of further disasters.

 > I forget what layout the disk has. Normally I used /dev/da0s1d to mount the disk.
 > What the next step should I do?

First, to see if there's any meaningful label there, try just:

# bsdlabel /dev/da0s1

If that disk just had a single 'd' partition, as sounds maybe likely?, 
and it doesn't show up with the above command, then you may get away 
with just making a fresh label for the whole slice, with just 'c' and 
'd' partitions, both of size 234436482 sectors.

I'm not the best person to advise on the right bsdlabel command, so I 
won't speculate (possibly mis-advising you) on that.  If you had other 
partitions than just 'd' in that slice then working out the boundaries 
and sizes will be a lot more complex, but if just one it looks fixable.

Of course, if the label at the beginning of slice 1 was clobbered, so 
may have been other data, but with a new label you might be able to 
recover it with 'fsck da0s1d' if the filesystem is more or less intact.

It's always worth keeping a copy of the output of bsdlabel for every 
FreeBSD slice somewhere safe (like on paper!) for times such as these.

cheers, Ian

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