On Wed, Jul 1, 2020 at 8:24 PM James Cassell
<fedoraproj...@cyberpear.com> wrote:
> On Wed, Jul 1, 2020, at 9:43 PM, Neal Gompa wrote:
> > On Wed, Jul 1, 2020 at 9:27 PM James Cassell

> > > Or maybe make all metadata raid 1, even on single disk set up?
> > >
> >
> > Not that isn't interesting, but what would be the mirror target on a
> > single disk setup?
> >
> The idea is that the second copy of metadata on the same disk might be 
> readable in case the first copy has a checksum error, in case of fault 
> hardware. I haven't tried it, but I'd gladly give up a little space for more 
> robustness, especially if btrfs is sensitive to metadata corruption by the 
> hardware. If btrfs demands a separate device for raid1 metadata, I wonder if 
> a small 1G partition could be dedicated for purely mirrored metadata use.

This is called 'dup' profile in Btrfs. Two copies of a block group. It
can be set on metadata only, or both metadata and data block groups.
It is the default mkfs option for HDDs. It is not enabled by default
on SSDs because concurrent writes of metadata i.e. they happen
essentially at the exact same time, means the data is likely to end up
on the same erase block, and typical corruptions affect the whole
block so it's widely considered to be pointless to use dup on flash
media. You can use it anyway, either with mkfs, or by converting the
block group from the single profile to dup. This is a safe procedure.

Chris Murphy
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