Chris Murphy <lists <at> colorremedies.com> writes: >>>> 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. > >> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
