Re: Growing 6 HDD RAID5 to 7 HDD RAID5

On 12 April 2011 22:15, NeilBrown [off-list ref] wrote:

On Wed, 13 Apr 2011 00:22:38 +0600 Roman Mamedov [off-list ref] wrote:

quoted

On Tue, 12 Apr 2011 18:21:13 +0100
Mathias Burén [off-list ref] wrote:

quoted

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 you specify --layout=preserve, then all the 'Q' blocks will be on one disk.
As every write needs to update a Q block, every write will write to that disk.

With our current RAID6 implementation that probably isn't a big cost - for
any write, we need to either read from or write to each disk anyway.

Anyway:  the only possible problem would be a performance problem, and I
really don't know what performance impact there is - if any.

quoted

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.

Note that doing a normalise by itself later will be much slower than not
doing a preserve now.
Doing the normalise later when growing the the device again would be just as
fast as no doing the preserve now.

NeilBrown

quoted

quoted

 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.

Right, so using --preserve seems like a sane and good option. Thanks
for the info, I'll let you know what happens, HDD should arrive the
next few days.

// Mathias
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