Had the joy of working on a new X5-2 with a 12c database recently. We added an ACFS mount and suddenly the ASM Operations showed a long re-sync process — days instead of minutes as usual. We asked around and no one we asked had seen this behavior. So after a little searching on the Oracle Support pages we found the bug and workaround in this note: “ASM RESYNC TOOK VERY LONG FOR ONE OF THE DISKGROUP IN EXADATA (Doc ID 1957331.1)”. Fixed it!
A few notes from the docs: You may want to make sure all of your files / DB’s are mounted and active before making this change (see the Oracle Doc: 12c ASM Docs )
We’re not sure if it was related, but before making this change startup and shutdown times on some empty default database(s) seemed inappropriately long, but now that seems to be cleared up as well(hmmm).