On Fri, 23 Nov 2012, free...@johnea.net wrote:

One of the complications was getting old metadata off of the drive. After 
trying a couple of 'dd' invocations:
# overwriting the first sector
dd if=/dev/zero of=/dev/ada0 bs=512 count=1
# also tried overwriting the last sector
diskinfo ada0 | cut -f4
(subtract 34, per WB) (I actually just subtracted the trailing 68)
dd if=/dev/zero of=/dev/ada0 seek=3907029100

This would still seem to not delete all of the metadata, since after issuing:

gmirror label -b split gm0 /dev/ada0
gmirror load
# repartition new mirror
gpart create -s MBR mirror/gm0
# ignore "mirror/gm0s1 added, but partition is not aligned on 4096 bytes" after 
gpart add -t freebsd -a 4k mirror/gm0
# create the bsdlabel partitions in slice 1 (s1)
gpart create -s BSD mirror/gm0s1

I would see that the old gm0s1a and gm0s1b had reappeared, even though I had 
not yet issued the 'add -t freebsd-ufs'. I'm not sure if they came back with 
the 'add -t freebsd' or the 'create -s BSD'.

Saved this since yesterday, thinking maybe I could come up with an idea, but so far I can't think what would cause that. It might not hurt to force a retaste after the dd.

The only thing that seemed to fix it was:

gpart destroy -F /dev/ada0

I also tried at one point:

gpart destroy -F ada0
gpart create -s gpt ada0
gpart destroy -F ada0

The thing I wonder about now: Should newfs include -S 4096?

I used:

newfs -U /dev/mirror/gm0s1a

Will this lead to 512 byte sector access to the disk through the file system?

Will this impact performance or longevity of the mirror?

It's a good question; I have not tried it.
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