Chris Murphy <lists <at> colorremedies.com> writes:

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6. mdadm on top of LVM2 LGs (not the other way around): 
would there be any issues or performance penalties?

You're not assured what PV the LV's are located on. 
...

Thanks for the advice, it makes a lot of sense. However, 
this question wasn't focused on the RAID6 itself, but 
related to some fancy (crazy?) mirroring scheme for the 
Linux partition I was considering: take a LV chunk from 
the VG that sits on the RAID6 and mirror (md RAID1) it 
with a partition from the SSD I'll be using for keeping 
the ext4 journal for the big data partition.

Sounds a bit nutty, no offense. It's complicated, non-standard, 
and therefore at high risk of user induced data loss.

It's basically raid61, which tells me you want the data 
always for sure always available. Because raid61 is about 
uptime. The problem is, you're not going to get that because 
you've overbuilt the storage stack and haven't considered 
(or mentioned) other fail points like the network, the power 
supply, power itself. So it just sounds wrongly overbuilt 
because the data can't possibly require this kind of uptime, 
chances are you're confusing raid with back ups. If the data 
is both important and it really needs to be available, build 
yourself a gluster cluster.

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In this way I can have a functional OS even if I take all 
the RAID6 disks offline. Of course, this can be achieved 
in other ways as well.

Well if everything you care about on this raid6 fits on an 
SSD partition, why don't you just set up an hourly rsync to 
the raid6 and use the SSD volume for live work? And then if 
you accidentally delete a file or crash when writing to the 
SSD chances are the states of the raid6 LV and the SSD volume 
are different and one is recoverable. If you raid1 them, any 
accidents affect both and you're hosed.

Thanks a lot for the advice, Chris. That's exactly what I hoped
for when posting to this list. As I mentioned, I am considering
a number what-if scenarios and possible solutions (I added the 
rsync one to that list) and weighting pros and cons. For this 
RAID61 approach, which seemed to make some logical sense, I had 
the feeling it's a bit fishy, but didn't have any real arguments 
against it. Now I have.

Since it seems you have a very healthy view of real-world RAID,
could you point out any significant issues when using a disk as
a degraded md RAID1 (not accidental, but on purpose)?

Vlad