Re: RAID5 won't mount after reducing disks from 8 to 6

[NeilBrown <neilb@suse.de>]

On Wed, 16 Feb 2011 20:47:08 -0500 Matt Tehonica <matt.tehonica@mac.com>
wrote:

> To be honest, I couldn't find good directions on how exactly to resize an array to be smaller so I put some bits and pieces together to do it.  Guess I didn't understand everything beforehand because I didn't read anything about resizing the file system.  I thought setting the "array-size" would take care of moving data before rebuilding onto the 6 disks.

The next release of mdadm will be a bit more explicit in the man page about
the need to resize the filesystem first - you aren't the first person to do
this.

> 
> Since I haven't resized the XFS filesystem, any recommendations on what to do next?  Think it's possible to recover any of the data?  For what it's worth, I haven't done anything to the 2 disks that I was going to remove.

8 to 6 disks in a RAID6 means 6 to 4 data disks, so 1/3 of the space is gone.
And it really is gone,  Little bits of it might be on the two devices that
you were going to remove, but I doubt it would be usable, and it would be
very hard to recover.

So your best bet it to convince xfs_repair to work with what you've got and
try to knit together as much as it can  - which may be nothing, I really
don't know.

Maybe you could ask on an XFS list somewhere.

But if you have any backups - I suggest they are by far your best bet.

I'm sorry, but it isn't really possible for mdadm to detect that you haven't
resized your filesystem and warn you about it :-(

NeilBrown