Re: [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO_VERSION field

[Jeff Layton]

On Mon, 2022-09-12 at 15:20 +0200, Florian Weimer wrote:
> * Jeff Layton:
> 
> > On Mon, 2022-09-12 at 14:13 +0200, Florian Weimer wrote:
> > > * Jeff Layton:
> > > 
> > > > To do this we'd need 2 64-bit fields in the on-disk and in-memory 
> > > > superblocks for ext4, xfs and btrfs. On the first mount after a crash,
> > > > the filesystem would need to bump s_version_max by the significant
> > > > increment (2^40 bits or whatever). On a "clean" mount, it wouldn't need
> > > > to do that.
> > > > 
> > > > Would there be a way to ensure that the new s_version_max value has made
> > > > it to disk? Bumping it by a large value and hoping for the best might be
> > > > ok for most cases, but there are always outliers, so it might be
> > > > worthwhile to make an i_version increment wait on that if necessary. 
> > > 
> > > How common are unclean shutdowns in practice?  Do ex64/XFS/btrfs keep
> > > counters in the superblocks for journal replays that can be read easily?
> > > 
> > > Several useful i_version applications could be negatively impacted by
> > > frequent i_version invalidation.
> > > 
> > 
> > One would hope "not very often", but Oopses _are_ something that happens
> > occasionally, even in very stable environments, and it would be best if
> > what we're building can cope with them.
> 
> I was wondering if such unclean shutdown events are associated with SSD
> “unsafe shutdowns”, as identified by the SMART counter.  I think those
> aren't necessarily restricted to oopses or various forms of powerless
> (maybe depending on file system/devicemapper configuration)?
> 
> I admit it's possible that the file system is shut down cleanly before
> the kernel requests the power-off state from the firmware, but the
> underlying SSD is not.
> 

Yeah filesystem integrity is mostly what we're concerned with here.

I think most local filesystems effectively set a flag in the superblock
that is cleared when the it's cleanly unmounted. If that flag is set
when you go to mount then you know there was a crash. We'd probably key
off of that in some way internally.
-- 
Jeff Layton <jlayton@xxxxxxxxxx>