On Jun 1, 2012, at 10:45 PM, Richard L. Hamilton wrote: > In a non-COW filesystem, one would expect that rewriting an already allocated > block would never fail for out-of-space (ENOSPC).
This seems like a rather broad assumption. It may hold for FAT or UFS, but might not hold for some of the more modern file systems (eg flash file systems) But I digress... > But I would expect that it could on ZFS - definitely if there was a snapshot > around, as it would create a divergence from that snapshot (because both > blocks would be kept). Or if deduplication was in effect, and the new block > contents were unique when the old contents hadn't been unique. > > Could rewriting a block _ever_ fail with ENOSPC if there _wasn't_ a snapshot > present, or is the replace old block with new somehow guaranteed to succeed, > so as to avoid introducing unexpected semantics? (say maybe there's a > reserved amount of free space just for rewrites to avoid that sort of > problem, or some other magic) There is a reserve at the pool level. It is needed for the ZIL at the very least. > I would think DBMS developers allowing databases to be stored on ZFS, as well > as folks using mmap(), might particularly want to be aware of the cases in > which an errno not anticipated from experience with other filesystems might > arise. Those developers have to handle all error conditions anyway. NB, many important databases are also COW, so the concept is well understood. -- richard -- ZFS Performance and Training richard.ell...@richardelling.com +1-760-896-4422 _______________________________________________ OpenIndiana-discuss mailing list OpenIndiana-discuss@openindiana.org http://openindiana.org/mailman/listinfo/openindiana-discuss
