Re: btrfs raid1 metadata, single data

[Sjoerd <sjoerd@sjomar.eu>]

On Friday 07 August 2015 19:18:02 Russell Coker wrote:
> On Fri, 7 Aug 2015 06:49:58 PM Robert Krig wrote:
> > What exactly is contained in btrfs metadata?
> 
> Much the same as in metadata for every other filesystem.
> 
> > I've read about some users setting up their btrfs volumes as
> > data=single, but metadata=raid1
> > 
> > Is there any actual benefit to that? I mean, if you keep your data as
> > single, but have multiple copies of metadata, does that still allow you
> > to recover from data corruption? Or is metadata redundancy a benefit to
> > ensure that your btrfs volume remains mountable/readable?
> 
> If you have redundant metadata and experience corruption then you will know
> the name of every file that has data corruption, this is really good for
> restoring from backup.  Also you will be protected against corruption of a
> root directory causing massive data loss.
> 
> If you have the bad luck to have certain metadata structures corrupted with
> no redundancy then you can face massive data loss and possibly have the
> entire filesystem become at least temporarily unusable.  While corruption
> of the root directory is unlikely it is possible.  With "dup" metadata I've
> seen a BTRFS filesystem remain usable after 12,000+ read errors.

While we're at it: any idea why the default for SSD's is single for meta data 
as described on the wiki? 
(https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices#Filesystem_creation)

I was looking for an answer why my SSD just had single metadata, while I 
expected it to be DUP and stumbled on this wiki article. Can't find a reason 
for why a SSD would be different?

Cheers,
Sjoerd