On 12/30/2016 02:39 AM, Mayavimmer wrote:
The plan is to nondestructively upgrade the old OS to F25 by keeping both for a while, giving the user the option to boot the old OS if they want
I don't understand how "upgrading" the old OS will be "non destructive" or give the user the option to boot the old system. Can you clarify? An upgrade is generally understood to be an in-place operation, in which case booting the old system would not be an option.
Meanwhile I would slowly move the old data over to new dynamic Btrfs raid1, to take advantage of its gorgeous scrub and autorepair mechanism.
As I understand it, those scrub and repair options really work best when btrfs is directly on top of the individual disk partitions. If you put stuff like md RAID underneath btrfs, it loses the ability to detect and repair some types of errors. For example, if you configure data mirroring / RAID1 in btrfs, it will write each data block to each disk with checksum. When you scrub the data, if there was a bit flip or other error, btrfs can determine which disk has the correct data and replace the block on the disk where the data is corrupt. If you have btrfs on top of an abstraction, like md RAID, then btrfs can only directly access one copy. If the copy is corrupt, btrfs can identify that as being the case, but it doesn't have the second copy to restore, so it can't fix the error. md RAID will do its own check and where it sees mismatching blocks, it doesn't know which is correct and can't reliably fix the data like btrfs can.
If you want to take advantage of it, you probably *really* want to build an entirely new system.
The installer is confusing to me. Logic would suggest that you choose LVM device, then select a Btrfs volume instead of ext4, but there is no such option!
I haven't verified that, but it wouldn't surprise me, for the reasons described above.
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