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

[Winder]

>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?
>How-To-Repeat:

>Fix:


>Release-Note:
>Audit-Trail:
>Unformatted: