[linux-lvm] pv recovery after trashing partition table.

[Jim N Cromie <james.cromie@juno.com>]

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Q:  how is 'dd' like motorcycles ?
A:  there are 2 types of users, those who have crashed, and those who
will.

Well, Ive now joined the ranks of folks who have.  I blew away the hda
partition table.

/dev/hda4 used to have a good PV.

Miraculously, Ive successfully recovered hda[1-3], by a combination of
luck,
a simple partition table (1=vfat, 2=swap, 3=ext2, 4=lvm, on cylinder
boundaries), good notes (I had the cylinder numbers mostly written down),
and perseverance with fdisk.  I wrote experimental partition-tables
untill I was able to mount them.

Just imagine my glee when hda3 fsck'd good !


So now, Im trying to recover hda4 now, with LVM volumes intact;

vgcfgrestore -l -l -v -d -n _vgb, reports 2 PVs,  hdb2, hda4,  both are
available, with some unusable, ( I cant cut-paste here, different
computer, cant boot clean without /var

pvscan however reports 2 different PVs,  hdb2, and hdc9

Im guessing that I did a pvcreate /dev/hdc9 after some snapshot/backup,
since hdc9 isnt there.  This suggests that my chances of recovery are
poorer than otherwize.  I never actually did a vgcfgsave - Ive only been
a couple of weeks with LVM, and it was only /tmp...

anyway, pvdata /dev/hda4 shows bogus (ie unfavorable, not wrong) data in
the PV, it also segfaults, $? is 139

PV size                8 / Not usable 262 GB [LVM 3.71 GB]
PV#                     0
PE Size(KB)        250
uuid                    none

btw - 3.71 GB is reasonable, the hda4 has ~3.9 GB

$> pvdisplay /dev/hda4
-- no physical volume identifier


vgcfgrestore gives error when I try above:

ERROR: pv_read(): PV identifer invalid" reading physical volume
/dev/hda4

I should mention; I dont understand how sectors in hda4 got corrupted,
at least by my dd ( I did have bs=512 count=1, and hda[1-3] are ok).
Something else may be missing..


SO, WHAT IF ANY ARE MY OPTIONS ?

1) Would a carefully considered set of 'dd' commands possibly
restore it?   I have nothing to lose in hda4 at this point.

2) IS IT POSSIBLE THAT DESPITE MY SUCCESS MOUNTING hda 1-3, particularly
hda3, since thats ext2, (ie a real filesystem, with a real fsck).  Ive
somehow got the partitions wrong, and thus pvdata is looking in the wrong
disk sector for its meta-data  ?

3) is it possible to use fdisk to change partition type to Linux, then do
an fsck ?
Im theorizing that fsck will find 3 different ext2 super-blocks somewhere
within the previously mountable LVM partition ?

I think I can claim enough space elsewhere, and make a file-clone of the
partition, if so, I can let fsck do all its repair attempts w/o trashing
the possibility
of using approaches 1,2


tia
jimc

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