New option for newfs(3) to make life with GEOM easier

[Bernd Walter]

On Sat, Sep 01, 2007 at 11:30:35AM +0200, Gergely CZUCZY wrote:
> On Sat, Sep 01, 2007 at 01:23:10PM +0400, Yar Tikhiy wrote:
> > On Sat, Sep 01, 2007 at 08:13:07AM +0000, Poul-Henning Kamp wrote:
> > > In message <20070901074803.GM85633 at comp.chem.msu.su>, Yar Tikhiy writes:
> > > >Hi all,
> > > >
> > > >With some geom(4) modules saving their metadata in the last sectors
> > > >of block devices such as disks and partitions, 
> > > 
> > > 1.  If those geom modules do not reduce their providers to prevent
> > > this metadata from being overwritten, they are buggy.
> > 
> > In some scenarios, it can be desirable to newfs first, geom later.
> True, done it many time myself. Since sysinstall doesn't allow you
> to install onto a gmirror array, many install via sysinstall, and gmirror
> the system afterwards, which is exactly this situation.

Sysinstall easily allows you to not partition the last few sectors.
The newfs option is only usefull if you are mirroring at fs level,
which is note quite common for system disks, where you really need 
partitions.

> Though for this, geom class manuals should mention how much
> space do they need for the metadata at the end. Or to simplify
> thing an option for like "reserve some space for (gmirror|gstripe|gfoobar)"
> should be introduced. Or specifing the module and newfs could "ask"
> the geom class for its metadata size that should be reserved.
> Just thinking, sorry if it was too wild...

And there is currently no way to find out if your calculations were
wrong, other than try and fail.
I guess it would be good if a mount tries to read the last sector and
gives a warning if the read fails.
I expect many systems were build by adding gmirror later without taking
the last sector into account.

-- 
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