Put simply, filesystems are the mechanisms used to read from and write to hard drives and other storage media. Mac users don’t tend to think about them much, because we haveo only one really common flavor: fire up Disk Utility and point it to pretty much any Mac-formatted disk, and odds are it’s in Mac OS Extended format.
Mac OS X Extended format is synonymous with “HFS+”, which has been around since System 8.1. HFS+ replaced HFS (Hierarchical File System), which in turn replaced MFS (Macintosh File System) waaaaay back in System 2. (On the original Macs, folders were just graphical representations in the Finder; every file on a disk was actually stored in one big folder beneath the hood.) Folks who have attached Windows-formatted drives to their Macs have probably already dealt with Windows filesystems: NTFS and the older FAT32.
HFS+ is getting fairly old and creaky in comparison to newer filesystems, so much buzz was generated when Apple expressed interest in ZFS, developed by Sun Microsystems, as a possible successor. ZFS
does things with disks which appear magical. For example, attach a new terabyte drive to a ZFS system, and you don’t get a new, separate volume; instead, you get a terabyte added to your existing volume. Time Machine-style backups are automatic without additional software, as well as disk snapshots similar to what’s made available by disk utilities like
Limited ZFS support debuted with Leopard, which could read ZFS disks, but some people were
still hoping for full-blown support in Snow Leopard. When it didn’t arrive in 10.6, the true believers still held out for it in a future release.
Which is not to say we’re stuck with HFS+ in perpetuity; Apple is apparently
hiring filesystem engineers as we speak. If HFS+ lasts as long as HFS did, then we’re due for an upgrade sometime in 2012.