On Sun 02-04-17 09:05:26, Dave Chinner wrote: > On Thu, Mar 30, 2017 at 12:12:31PM -0400, J. Bruce Fields wrote: > > On Thu, Mar 30, 2017 at 07:11:48AM -0400, Jeff Layton wrote: > > > On Thu, 2017-03-30 at 08:47 +0200, Jan Kara wrote: > > > > Because if above is acceptable we could make reported i_version to be a sum > > > > of "superblock crash counter" and "inode i_version". We increment > > > > "superblock crash counter" whenever we detect unclean filesystem shutdown. > > > > That way after a crash we are guaranteed each inode will report new > > > > i_version (the sum would probably have to look like "superblock crash > > > > counter" * 65536 + "inode i_version" so that we avoid reusing possible > > > > i_version numbers we gave away but did not write to disk but still...). > > > > Thoughts? > > > > How hard is this for filesystems to support? Do they need an on-disk > > format change to keep track of the crash counter? > > Yes. We'll need version counter in the superblock, and we'll need to > know what the increment semantics are. > > The big question is how do we know there was a crash? The only thing > a journalling filesystem knows at mount time is whether it is clean > or requires recovery. Filesystems can require recovery for many > reasons that don't involve a crash (e.g. root fs is never unmounted > cleanly, so always requires recovery). Further, some filesystems may > not even know there was a crash at mount time because their > architecture always leaves a consistent filesystem on disk (e.g. COW > filesystems).... What filesystems can or cannot easily do obviously differs. Ext4 has a recovery flag set in superblock on RW mount/remount and cleared on umount/RO remount. This flag being set on mount would imply incrementing the crash counter. It should be pretty easy for each filesystem to implement such flag and the counter but I agree it requires an on-disk format change. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
