Re: Growing 6 HDD RAID5 to 7 HDD RAID5

[Roman Mamedov <rm@romanrm.ru>]

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On Tue, 12 Apr 2011 18:21:13 +0100
Mathias Burén <mathias.buren@gmail.com> wrote:

> If I use --layout=preserve , what impact will that have?
> If I preserve the layout, what is the final result of the array
> compared to not preserving it?

Neil wrote about this on his blog:
"It is a very similar process that can now be used to convert a RAID5 to a
RAID6. We first change the RAID5 to RAID6 with a non-standard layout that has
the parity blocks distributed as normal, but the Q blocks all on the last
device (a new device). So this is RAID6 using the RAID6 driver, but with a
non-RAID6 layout. So we "simply" change the layout and the job is done."
http://neil.brown.name/blog/20090817000931

Admittedly it is not completely clear to me what are the long-term downsides of
this layout. As I understand it does fully provide the RAID6-level redundancy.
Perhaps just the performance will suffer a bit? Maybe someone can explain this
more.

If anything, I think it is safe to use this layout for a while, e.g. in case
you don't want to rebuild 'right now'. You can always change the layout to the
traditional one later, by issuing "--grow --layout=normalise". Or perhaps if
you plan to add another disk soon, you can normalise it on that occasion, and
still gain the benefit of only one full reshape.

>  Will the array have redundancy during the rebuild of the new drive?

If you choose --layout=preserve, your array immediately becomes a RAID6 with
one rebuilding drive. So this is the kind of redundancy you will have during
that rebuild - tolerance of up to one more (among the "old" drives) failure,
in other words, identical to what you currently have with RAID5.

-- 
With respect,
Roman

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