Re: Confused about reshape?

[Neil Brown <neilb@suse.de>]

On Sun, 30 May 2010 18:29:16 -0500
"Leslie Rhorer" <lrhorer@satx.rr.com> wrote:

> 
> 	Am I confused about reshaping a RAID5 to RAID6 array, or is there an
> error in the man page?  The distro I have on one of my servers - Debian
> "Squeeze" - employs mdadm 3.0.3, internally dated Oct. 22, 2009.  The man
> page that comes with the software specifically says:
> 
> -l, --level=
> 
> <snip>
> 
> 	Not yet supported with --grow.

Man page is wrong.  Thanks for reporting.  Fixed in current .git and will be
in next release.

> 
> 	Searching the web, however, I find the following link from Aug. 17,
> 2009:
> 
> http://neil.brown.name/blog/20090817000931
> 
> 	In the link, it says:
> 
> Now, in mid 2009, we have most of the rest of the reshaping options that had
> been planned. These include changing the stripe size, changing the layout
> (i.e. where the parity blocks get stored) and reducing the number of
> devices.
> 
> Changing the layout provides valuable functionality as it is an important
> part of converting a RAID5 to a RAID6.
> 
> <snip>
> 
> Currently supported reshapes
> 
> <snip>
> 
> RAID5
> A RAID5 can change the number of devices, the size of the individual
> devices, the chunk size and the layout. A 2 drive RAID5 can be converted to
> RAID1, and a 3 or more drive RAID5 can be converted to RAID6.
> 
> 	Since the 3.0.3 version of mdadm was evidently written in October,
> and since this link was from August, one would think 3.0.3 should be able to
> re-shape a RAID5 to RAID6.  If so, the man page (at least for Debian
> "Squeeze") is incorrect, and of course fails to offer the syntax for such an
> upgrade.
> 
> 	Am I missing something?

The syntax for changing levels is simply:

  mdadm --grow /dev/mdX --level=6

This will preserve the number of data disks.  If you want to change that (or
anything else) at the same time, you can:

 mdadm --grow /dev/mdX --level=6 --raid-disk=8 --chunk-size=1024 --layout=left-asymetric

Thanks,
NeilBrown


> 
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