Re: Growing 6 HDD RAID5 to 7 HDD RAID5

[NeilBrown <neilb@suse.de>]

[-- Attachment #1: Type: text/plain, Size: 2527 bytes --]

On Wed, 13 Apr 2011 00:22:38 +0600 Roman Mamedov <rm@romanrm.ru> wrote:

> 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 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.

> 
> 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


> 
> >  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.
> 


[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 190 bytes --]