A very interesting article wrotete by Jeff Bonwick for Andrew -- "Rampant Layering Violation?" http://blogs.sun.com/bonwick/entry/rampant_layering_violation 2008/12/18 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>: > 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-btrfs" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- Thanks & Best Regards Liu Hui -- -- 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
