docs/128804: Gmirror handbook instructions convert boot-drive to gmirror, truncating last sector

[Tom Rhodes]

The following reply was made to PR docs/128804; it has been noted by GNATS.

From: Tom Rhodes <trhodes at FreeBSD.org>
To: Winder <freebsdbugs.20.awinder at spamgourmet.com>
Cc: freebsd-gnats-submit at FreeBSD.org
Subject: Re: docs/128804: Gmirror handbook instructions convert boot-drive
 to gmirror, truncating last sector
Date: Wed, 12 Nov 2008 02:18:59 -0500

 On Wed, 12 Nov 2008 01:21:55 GMT
 Winder <freebsdbugs.20.awinder at spamgourmet.com> wrote:
 
 Hi,
 
 
 > 
 > >Number:         128804
 > >Category:       docs
 > >Synopsis:       Gmirror handbook instructions convert boot-drive to gmirror, truncating last sector
 > >Confidential:   no
 > >Severity:       non-critical
 > >Priority:       medium
 > >Responsible:    freebsd-doc
 > >State:          open
 > >Quarter:        
 > >Keywords:       
 > >Date-Required:
 > >Class:          doc-bug
 > >Submitter-Id:   current-users
 > >Arrival-Date:   Wed Nov 12 01:30:01 UTC 2008
 > >Closed-Date:
 > >Last-Modified:
 > >Originator:     Winder
 > >Release:        7.1-PRERELEASE
 > >Organization:
 > >Environment:
 > FreeBSD DEV2 7.1-PRERELEASE FreeBSD 7.1-PRERELEASE #3: Fri Nov  7 19:54:41 MST 2008     windrider at DEV2:/usr/obj/usr/src/sys/packetcheckv1-fileserv  amd64
 > >Description:
 > http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/geom-mirror.html
 > 
 > This geom mirroring documentation shows a way to convert the boot drive into a gmirror raid. This guide misleads in assuming your whole drive has not been used.
 > 
 > The problem is that gmirror truncates the last sector of the drive for metadata; thus, any data stored in that sector when adding the drive into the RAID is lost. Sysinstall uses the entire drive by default. Following these instructions, then, could cause loss of data or a corrupt partition, as it will reach one sector beyond the end of the geom mirror device.
 > 
 > mightn't it be better to make an example from a blank disk?
 
 I would think that converting the disk device to a software
 RAID, at least the boot disk, would be the way most people
 wanted to use it.  And in most cases, I think that it's one
 of the first things done during an installation - though,
 YMMV.  If the last sector is filled with user data - there
 are larger problems, possibly, than just where GEOM places
 metadata.
 
 If you feel a warning is required here, I'll gladly add one,
 but it never crossed my mind initially that someone would
 actually be using the last sector on the drive for data.
 
 Thanks,
 
 -- 
 Tom Rhodes