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

[Tom Rhodes]

On Wed, 12 Nov 2008 20:04:54 -0500
Tom Rhodes <trhodes at FreeBSD.org> wrote:

> On Wed, 12 Nov 2008 09:54:49 -0500
> "Bob Johnson" <fbsdlists at gmail.com> wrote:
> 
> > 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
> 
> 
> Don't Unix file systems attempt to lump data closer together
> in order to reduce this fragmentation which scatters data
> across the entire drive, making it unlikely to have data in
> that last sector unless one went beyond the 8% boundry?

Sorry, I really hope this didn't come off as rude.  While I know
the user has no control over where data is stored on the hard
disk, I would also assume a warning about this to be in the
manual page as well.  It's very possible that the metadata is
stored in a location not touchable by a filesystem.  Thus, I
have sent an email to Pawel for further examination.  Thanks,


> 
> 
> > 
> > 
> > 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
> > > _______________________________________________
> > > freebsd-doc at freebsd.org mailing list
> > > http://lists.freebsd.org/mailman/listinfo/freebsd-doc
> > > To unsubscribe, send any mail to "freebsd-doc-unsubscribe at freebsd.org"
> > >
> > 
> > 
> > -- 
> > -- Bob Johnson
> >    fbsdlists at gmail.com
> > _______________________________________________
> > freebsd-doc at freebsd.org mailing list
> > http://lists.freebsd.org/mailman/listinfo/freebsd-doc
> > To unsubscribe, send any mail to "freebsd-doc-unsubscribe at freebsd.org"
> > 
> 
> 
> -- 
> Tom Rhodes
> 


-- 
Tom Rhodes