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?
RAID6 is intended to be tolerant of two disk failures. In the case of
there being a double failure and only 5 disks, the ease with which the
user can balance/convert to a 3-disk raid5 is also important.
Please shoot down my concerns. :)
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Brendan Hide
http://swiftspirit.co.za/
http://www.webafrica.co.za/?AFF1E97
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