gmirror on existing filesystem (was Fresh install on gmirror'ed disks?)

[Pawel Jakub Dawidek]

On Tue, Apr 04, 2006 at 05:51:26PM -0400, Mike Jakubik wrote:
+> >>>Can someone with knowledge of the UFS internals please confirm one way
+> >>>or the other if this is dangerous or not?
+> >>>  
+> >>
+> >>I'm curious to know this as well, as i have some systems using gmirror, that were setup in this fashion. Could someone knowledgeable on the matter shed some light?
+> >
+> >
+> >I've gmirrored existing disks/slices before, and it's worked fine.  I'm not 100% certain about all cases, but it's possible that the filesystem could be right up against 
+> >the last block of the partition, and it could get stomped on I suppose.
+> >
+> >I'm not sure what this command tells you for sure, but it dumps the last block of a slice, or disk, or whatever:
+> >
+> >
+> >dd if=/dev/ad0s3a iseek=`diskinfo ad0s3a | perl -ne '@d = split; print ($d[2]/$d[1] - 1)'` count=512 | hexdump
+> 
+> Could someone provide an authoritative answer to this please? Pawel, it would be nice to see some support for your own code from you. This is a very easy method to create a 
+> mirror on an existing system, but if its going to cause problems then its useless (All the more reason for geom enabled installer).

I can't give you an authoritative answer, because I don't know UFS
internals so well. All I know is that it (UFS) thinks the last sector is
available and may want to use it at some point getting EIO then.
I'm not using this method, but I've heard of many people using it
without problems.

Speaking about installer. I don't think I'll be able to add
configuration of my GEOM classes to the sysinstall in the near future
(and I hope never - I'd prefer to wait for a new installer).
One can still see how many sectors exactly has the partition he is going
to create file system on and add additional newfs(8) flag
'-s <sectors-1>'.

-- 
Pawel Jakub Dawidek                       http://www.wheel.pl
pjd at FreeBSD.org                           http://www.FreeBSD.org
FreeBSD committer                         Am I Evil? Yes, I Am!
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