Dave wrote:
Has this been discussed here? Has anything changed since it was written?
I have (more or less) been following the mailing list since this feature
was suggested. I have been drooling over it since, but not much have
happened.
Parity-based redundancy (RAID5/6/triple parity and beyond) on BTRFS
and MDADM (Dec 2014) – Ronny Egners Blog
http://blog.ronnyegner-consulting.de/2014/12/10/parity-based-redundancy-raid56triple-parity-and-beyond-on-btrfs-and-mdadm-dec-2014/comment-page-1/
TL;DR: There are patches to extend the linux kernel to support up to 6
parity disks but BTRFS does not want them because it does not fit
their “business case” and MDADM would want them but somebody needs to
develop patches for the MDADM component. The kernel raid
implementation is ready and usable. If someone volunteers to do this
kind of work I would support with equipment and myself as a test
resource.
--
I am just a list "stalker" and no BTRFS developer, but as others have
indirectly said already. It is not so much that BTRFS don't want the
patches as it is that BTRFS do not want to / can't focus on this right
now due to other priorities.
There was some updates to raid5/6 in kernel 4.12 that should fix (or at
least improve) scrub/auto-repair. The write hole does still exist.
That being said there might be configurations where btrfs raid5/6 might
be of some use. I think I read somewhere that you can set data to
raid5/6 and METADATA to raid1 or 10 and you would risk loosing some data
(but not the filesystem) in the event of a system crash / power failure.
This sounds tempting since it in theory would not make btrfs raid 5/6
significantly less reliable than other RAID's which will corrupt your
data if the disk happens to spits out bad bits without complaining (one
possible exception that might catch this is md raid6 which I use). That
being said there is no way I would personally use btrfs raid 5/6 even
with metadata raid1/10 yet without proper tested backups at standby at
this point.
Anyway - I would worry more about getting raid5/6 to work properly
before even thinking about multi-parity at all :)
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