Re: In this partition scheme, grub does not find md information?

["Keld Jørn Simonsen" <keld@dkuug.dk>]

On Tue, Jan 29, 2008 at 07:46:58PM +0300, Michael Tokarev wrote:
> Keld Jørn Simonsen wrote:
> > On Tue, Jan 29, 2008 at 06:13:41PM +0300, Michael Tokarev wrote:
> >> Linux raid10 MODULE (which implements that standard raid10
> >> LEVEL in full) adds some quite.. unusual extensions to that
> >> standard raid10 LEVEL.  The resulting layout is also called
> >> raid10 in linux (ie, not giving new names), but it's not that
> >> raid10 (which is again the same as raid1+0) as commonly known
> >> in various literature and on the internet.  Yet raid10 module
> >> fully implements STANDARD raid10 LEVEL.
> > 
> > My understanding is that you can have a linux raid10 of only 2
> > drives, while the standard RAID 1+0 requires 4 drives, so this is a huge
> > difference.
> 
> Ugh.  2-drive raid10 is effectively just a raid1.  I.e, mirroring
> without any striping. (Or, backwards, striping without mirroring).

OK.  

uhm, well, I did not understand: "(Or, backwards, striping without
mirroring)."  I don't think a 2 drive vanilla raid10 will do striping.
Please explain.

> Pretty much like with raid5 of 2 disks - it's the same as raid1.

I think in raid5 of 2 disks, half of the chunks are parity chynks which
are evenly distributed over the two disks, and the parity chunk is the
XOR of the data chunk. But maybe I am wrong. Also the behaviour of suce
a raid5 is different from a raid1 as the parity chunk is not used as
data.
> 
> > I am not sure what vanilla linux raid10 (near=2, far=1)
> > has of properties. I think it can run with only 1 disk, but I think it
> 
> number of copies should be <= number of disks, so no.

I have a clear understanding that in a vanilla linux raid10 (near=2, far=1)
you can run with one failing disk, that is with only one working disk.
Am I wrong?

> > does not have striping capabilities. It would be nice to have more 
> > info on this, eg in the man page. 
> 
> It's all in there really.  See md(4).  Maybe it's not that
> verbose, but it's not a user's guide (as in: a large book),
> after all.

Some man pages have examples. Or info could be written in the faq or in
wikipedia.

Best regards
keld
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