Create a mirror on disk with valid data

[Victor Sudakov]

Danny Howard wrote:
> > > 
> > > I have only ever mirrored disks with data on them.  Its a question of
> > > bootstrap - does the mirror comes first or does the data you are going
> > > to mirror come first?
> > 
> > Suppose the disk has valuable data in the last sector and you are
> > going to create a mirror from this disk. What is going to happen when
> > the last sector is overwritten with the mirror metadata? Your data will
> > be lost, right? Suppose you need to access the last sector, access
> > will be denied, right?
> 
> >From what I have read, the gmirror stuff uses a sector of the disk that
> isn't used at the end, and if you format a disk in such a way that that
> free sector is unavailable, you can not set up a gmirror.

Where have you read this? Where is it documented? I would like to read
this too.

I provided some screen output, do you mean to say I have formatted the
disk incorrectly prior to mirroring it?

> 
> If you find a scenario in which setting up a gmirror destroys data on a
> disk, then please use send-pr to file a bug, so that that issue can be
> fixed.

I have found a scenario in which a gmirror prevents a partition from
being newfs'ed. Is this enough for a PR ?

> 
> But, if your question is "does gmirror do evil things to existing data"
> then I'll answer that I and others have gmirrored quite a few disks with
> existing data 

Have you and others tried to access the last sector of those disks?
For example, with dd? May I ask you to try?

> and I'm not aware that anybody has been bitten by your
> hypothesis.

Have you looked at the screen output I provided? What was I doing wrong?

-- 
Victor Sudakov,  VAS4-RIPE, VAS47-RIPN