Re: Growing 6 HDD RAID5 to 7 HDD RAID5

["Mathias Burén" <mathias.buren@gmail.com>]

On 12 April 2011 22:15, NeilBrown <neilb@suse.de> wrote:
> 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.
>>
>
>

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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