[Discuss] btrfs

[dsr at randomstring.org (Dan Ritter)]

On Fri, Feb 22, 2013 at 12:08:32PM -0500, Derek Atkins wrote:
> Rich Pieri <richard.pieri at gmail.com> writes:
> 
> > On Fri, 22 Feb 2013 11:29:42 -0500
> > Jerry Feldman <gaf at blu.org> wrote:
> >
> >> So, assume I have 2 physical volumes, /dev/sda and /dev/sdb.
> >> mkfs.btrfs -d raid1 /dev/sda /dev/sdb
> >> What happens if I get a failure on /dev/sdb.
> >> Assume no snapshots.
> >
> > "-d raid1" means mirrored data. Metadata is mirrored by default even
> > on single drive volumes.
> >
> > If /dev/sdb faults then you should lose no data since every extent is
> > replicated on both /dev/sda and /dev/sdb. If a bit error arises on
> > either sda or sdb then a scrub will detect the error and it should
> > automatically correct it using the replica on the other device.
> 
> I'm sure these are silly questions I could google myself, but: what
> happens with more than 2 devices?  For example, if I used:
> 
>   mkfs.btrfs -d raid1 /dev/sda /dev/sdb /dev/sdc /dev/sdd
> 
> It this going to be more like raid10?

No, that's still RAID1: two copies of every file, no striping.
If you want striping+mirroring, turn on -d RAID10. 

> Also, can you add new devices "later" to an existing FS?  E.g., let's
> say we start with 2 devices (sda, sdb) -- can I later add more devices?

Yes, and you can convert between RAID0, 1 and 10 in a
live-but-slow fashion.

-dsr-