Messages in this thread | | | Date | Sun, 27 Jun 1999 10:08:11 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [patch] pagecache-2.3.9-H3, bmap & ext2fs cleanup patch |
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On Sun, 27 Jun 1999, Ingo Molnar wrote: > > no, RAID is really a different concept, and it's filesystem independent. > multi-device filesystems _might_ be useful but the cleanest way of > handling striping and/or redundancy is IMO MD at the block device layer. > Caching and block allocation decisions are best done on the filesystem > level.
The best argument for having the filesystem know about devices is to have filesystems that can "migrate" files from one device to another. It can be done other ways too, but basically it is really nice for "intelligent" filesystems that do (for example) dynamic load balancing or that have multiple levels of caching.
Basically I'm talking about hierarchical filesystems: everything looks like one large filesystem, but the filesystem itself can dynamically move things from a slow medium to a fast medium according to usage patterns. Old stuff that is seldom used is on a slow disk (or disk arrays), while stuff that is used regularly is on a fast one.
I'm not writing one, and I don't know of anybody _else_ writing one either, but this kind of schenario is why I think the VFS layer should not ever assume that we have just one device. So when we're cleaning up other assumptions, we're better off cleaning this one up too - especially as it's so simple and so "obvious" (the "Mapped" attribute really means that both the block number and the device is known, so it makes much more conceptual sense to have the mapper do both).
Linus
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