On Wed, 17 Dec 2008 08:23:44 -0500 Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > FYI: here's a little writeup I did this summer on support for > filesystems spanning multiple block devices: > > > -- > > === Notes on support for multiple devices for a single filesystem === > > == Intro == > > Btrfs (and an experimental XFS version) can support multiple underlying block > devices for a single filesystem instances in a generalized and flexible way. > > Unlike the support for external log devices in ext3, jfs, reiserfs, XFS, and > the special real-time device in XFS all data and metadata may be spread over a > potentially large number of block devices, and not just one (or two) > > > == Requirements == > > We want a scheme to support these complex filesystem topologies in way > that is > > a) easy to setup and non-fragile for the users > b) scalable to a large number of disks in the system > c) recoverable without requiring user space running first > d) generic enough to work for multiple filesystems or other consumers > > Requirement a) means that a multiple-device filesystem should be mountable > by a simple fstab entry (UUID/LABEL or some other cookie) which continues > to work when the filesystem topology changes. "device topology"? > Requirement b) implies we must not do a scan over all available block devices > in large systems, but use an event-based callout on detection of new block > devices. > > Requirement c) means there must be some version to add devices to a filesystem > by kernel command lines, even if this is not the default way, and might require > additional knowledge from the user / system administrator. > > Requirement d) means that we should not implement this mechanism inside a > single filesystem. > One thing I've never seen comprehensively addressed is: why do this in the filesystem at all? Why not let MD take care of all this and present a single block device to the fs layer? Lots of filesystems are violating this, and I'm sure the reasons for this are good, but this document seems like a suitable place in which to briefly decribe those reasons. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
