On Tue, May 12, 2015 at 07:49:02AM +1000, Dave Chinner wrote: > On Mon, May 11, 2015 at 11:52:52AM -0500, Eric Sandeen wrote: > > On 5/11/15 11:14 AM, Brian Foster wrote: > > > On Fri, May 08, 2015 at 03:48:16PM -0500, Eric Sandeen wrote: > > >> > This adds a new superblock field, sb_meta_uuid. If set, along with > > >> > a new incompat flag, the code will use that field on a V5 filesystem > > >> > to compare to metadata UUIDs, which allows us to change the user- > > >> > visible UUID at will. Userspace handles the setting or clearing > > >> > of the incompat flag as appropriate, as the UUID gets written. > > >> > > > >> > If the incompat flag is not set, copy the user-visible UUID into > > >> > into the meta_uuid slot in memory; this is not written back to disk in > > >> > this case. > > >> > > > >> > The remainder of this patch simply switches verifiers, initializers, > > >> > etc to use the new sb_meta_uuid field. > > >> > > > >> > Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx> > > >> > --- > > > This all looks fine to me. The only thing that confuses me is why we > > > continue to use sb_uuid for the log. Eric points out on irc that we > > > can't modify the uuid when the log is dirty, so we can't actually break > > > anything in this manner. In other words, we effectively already handle a > > > modified uuid with respect to the log. > > > > > > That said, it seems inconsistent to me to create a metadata uuid field > > > and not use it for all metadata. My expectation from such a change is to > > > see sb_uuid now used only for user facing bits (e.g., mount and get geo > > > related) and sb_meta_uuid used everywhere else. Instead, we use sb_uuid > > > for the user facing bits, sb_meta_uuid for the internal bits that can't > > > handle a uuid change and sb_uuid for the internal bits that can. > > > > > > I suppose I could be convinced otherwise if there's context I'm missing, > > > but otherwise this seems a bit confusing to me... > > > > > > Brian > > > > Yeah, I'm sympathetic to that argument. I can look into using sb_meta_uuid > > for the log too, if the incompat flag is set. > > The log uuid is user facing, like the superblock uuid. i.e. there > are tools that identify external log devices by checking that the > uuid in the log matches the user facing uuid in the superblock. > Hence the log uuid needs to use the uuid that is reported to > userspace. > Ok, sounds good. Thanks for the explanation. Brian > > I had skipped it just because that use of the UUID predated the CRC changes. > > Changing the uuid results in the log being zeroed, so the uuid in > the log can be changed without issue, even if we are using CRCs. > > Cheers, > > Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx > > _______________________________________________ > xfs mailing list > xfs@xxxxxxxxxxx > http://oss.sgi.com/mailman/listinfo/xfs _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
