Re: raid6 + hot spare question

[Chris Murphy <lists@colorremedies.com>]

On Wed, Sep 9, 2015 at 9:48 AM, Brendan Hide <brendan@swiftspirit.co.za> wrote:
> Things can be a little more nuanced.
>
> First off, I'm not even sure btrfs supports a hot spare currently. I haven't
> seen anything along those lines recently in the list - and don't recall
> anything along those lines before either. The current mention of it in the
> Project Ideas page on the wiki implies it hasn't been looked at yet.
>
> Also, depending on your experience with btrfs, some of the tasks involved in
> fixing up a missing/dead disk might be daunting.
>
> See further (queries for btrfs-devs too) inline below:
>
> On 2015-09-08 14:12, Hugo Mills wrote:
>>
>> On Tue, Sep 08, 2015 at 01:59:19PM +0200, Peter Keše wrote:
>>>
>>> <snip>
>>> However I'd like to be prepared for a disk failure. Because my
>>> server is not easily accessible and disk replacement times can be
>>> long, I'm considering the idea of making a 5-drive raid6, thus
>>> getting 12TB useable space + parity. In this case, the extra 4TB
>>> drive would serve as some sort of a hot spare.
>
> From the above I'm reading one of two situations:
> a) 6 drives, raid6 across 5 drives and 1 unused/hot spare
> b) 5 drives, raid6 across 5 drives and zero unused/hot spare
>>>
>>>
>>> My assumption is that if one hard drive fails before the volume is
>>> more than 8TB full, I can just rebalance and resize the volume from
>>> 12 TB back to 8 TB essentially going from 5-drive raid6 to 4-drive
>>> raid6).
>>>
>>> Can anyone confirm my assumption? Can I indeed rebalance from
>>> 5-drive raid6 to 4-drive raid6 if the volume is not too big?
>>
>>     Yes, you can, provided, as you say, the data is small enough to fit
>> into the reduced filesystem.
>>
>>     Hugo.
>>
> This is true - however, I'd be hesitant to build this up due to the current
> process not being very "smooth" depending on how unlucky you are. If you
> have scenario b above, will the filesystem still be read/write or read-only
> post-reboot? Will it "just work" with the only requirement being free space
> on the four working disks?


There isn't even a need to rebalance, dev delete will shrink the fs
and balance. At least that's what I'm seeing here, and found a failure
in a really simple (I think) case, which I just made a new post about:
http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg46296.html

This should work whether on a failed/missing disk, or normally
operating volume so long as a.) the removal doesn't go below the
minimum devices and b.) there's enough space for the data as a result
of the volume shrink operation.



-- 
Chris Murphy