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

[Bob Johnson]

The user has no control over whether the last sector of the drive is
used for data. Data is scattered all over the drive, in something that
resembles a random fashion, and I know of no reason the last sector is
not just as likely to have data stored to it as any other sector on
the drive (but I am certainly not an authority on the filesystem
algorithms).

I think a warning is very much in order (that there is a small chance
that data will be lost by this procedure).

- Bob


On 11/12/08, Tom Rhodes <trhodes at freebsd.org> wrote:
> 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
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-- 
-- Bob Johnson
   fbsdlists at gmail.com