Re: [RFC] xfs: opting in or out of online repair

[Christoph Hellwig <hch@lst.de>]

On Thu, Jul 25, 2024 at 12:05:26PM +1000, Dave Chinner wrote:
> Maybe I'm missing something important - this doesn't feel like
> on-disk format stuff. Why would having online repair enabled make
> the fileystem unmountable on older kernels?

Yes, that's the downside of the feature flag.

> Hmmm. Could this be implemented with an xattr on the root inode
> that says "self healing allowed"?

The annoying thing about stuff in the public file system namespace
is that chowning the root of a file system to a random user isn't
that uncommon, an that would give that user more privileges than
intended.  So it could not hust be a normal xattr but would have
to be a privileged one, and with my VFS hat on I'd really like
to avoid creating all these toally overloaded random non-user
namespace xattrs that are a complete mess.

One option would be an xattr on the metadir root (once we merge
that, hopefully for 6.12).  That would still require a new ioctl
or whatever interface to change (or carve out an exception to
the attr by handle interface), but it would not require kernel
and tools to fully understand it.

> > Note that administrator-initated scans (e.g. invoking xfs_scrub from the
> > CLI) would not be blocked by this flag.
> > 
> > Question: Should this compat flag control background scrubs as well?
> 
> Probably. scrub is less intrusive, but I can see people wanting to
> avoid it because it can have a perf impact. Could this be done with
> a different xattr on the root inode?

Yes, scrub vs repair should probably be separate.