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[Discuss] btrfs



On Fri, Feb 22, 2013 at 11:29:42AM -0500, Jerry Feldman wrote:
> On 02/22/2013 11:01 AM, Rich Pieri wrote:
> > On Fri, 22 Feb 2013 10:04:24 -0500
> > Jerry Feldman <gaf at blu.org> wrote:
> >
> >> Most of the examples I have seen are to install btrfs on raw drives.
> > Btrfs is, like ZFS, both file system and volume manager. There is
> > typically no benefit to not allowing Btrfs to manage entire devices
> > unless you need to have part of the disk not be Btrfs. Not allowing
> > Btrfs to manage whole devices makes it more difficult to replace
> > faulted devices.
> >
> >> redundant,, but your data is essentially stripped (RAID0) so you
> >> effectively get more storage with the safety of RAID1. (You can
> >> configure btrfs to be fully redundant if you want to).
> > No. RAID0 means a device failure equals data loss. Mirrored metadata
> > will not save you from that. What mirrored metadata gets you is a
> > measure of protection against bit errors damaging file names,
> > permissions, checksums and related information.
> >
> > Note that Btrfs mirrors data and metadata, not disks or disk blocks. A
> > three-disk Btrfs raid1 is not three copies of every file extent. It is
> > two copies of every file extent stored one each on two out of the three
> > disks.
> 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.

Snapshots are irrelevant.

-m raid1 is default, and you have added -d raid1, so everything
is mirrored. When /dev/sdb drops out, you get an error message
and everything continues working. Replace the disk and continue.

-dsr-



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