On 06/03/16 20:59, Christoph Anton Mitterer wrote:
On Fri, 2016-06-03 at 13:42 -0500, Mitchell Fossen wrote:
Thanks for pointing that out, so if I'm thinking correctly, with
RAID1
it's just that there is a copy of the data somewhere on some other
drive.

With RAID10, there's still only 1 other copy, but the entire
"original"
disk is mirrored to another one, right?
As Justin mentioned, btrfs doesn't raid whole disks/devices. Instead, it works with chunks.


To be honest, I couldn't tell you for sure :-/ ... IMHO the btrfs
documentation has some "issues".

mkfs.btrfs(8) says: 2 copies for RAID10, so I'd assume it's just the
striped version of what btrfs - for whichever questionable reason -
calls "RAID1".

The "questionable reason" is simply the fact that it is, now as well as at the time the features were added, the closest existing terminology that best describes what it does. Even now, it would be difficult on the spot adequately to explain what it means for redundancy without also mentioning "RAID".

Btrfs does not raid disks/devices. It works with chunks that are allocated to devices when the previous chunk/chunk-set is full.

We're all very aware of the inherent problem of language - and have discussed various ways to address it. You will find that some on the list (but not everyone) are very careful to never call it "RAID" - but instead raid (very small difference, I know). Hugo Mills previously made headway in getting discussion and consensus of proper nomenclature. *

Especially, when you have an odd number devices (or devices with
different sizes), its not clear to me, personally, at all how far that
redundancy actually goes respectively what btrfs actually does... could
be that you have your 2 copies, but maybe on the same device then?

No, btrfs' raid1 naively guarantees that the two copies will *never* be on the same device. raid10 does the same thing - but in stripes on as many devices as possible.

The reason I say "naively" is that there is little to stop you from creating a 2-device "raid1" using two partitions on the same physical device. This is especially difficult to detect if you add abstraction layers (lvm, dm-crypt, etc). This same problem does apply to mdadm however.

Though it won't necessarily answer all questions about allocation, I strongly suggest checking out Hugo's btrfs calculator **

I hope this is helpful.

* http://comments.gmane.org/gmane.comp.file-systems.btrfs/34717 / https://www.spinics.net/lists/linux-btrfs/msg33742.html
* http://comments.gmane.org/gmane.comp.file-systems.btrfs/34792
** http://carfax.org.uk/btrfs-usage/



Cheers,
Chris.


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