Quoting Theodore Y. Ts'o (tytso@MIT.EDU):
> I've never claimed that the ext2 is the best way to do RAID; I think MD
> is the way to do that. However, allowing ext2 to be able to support
> filesystems which span multiple block devices is a good thing to do, and
> a cleaner way of supporting multivolume support. Examples of
> filesystems which do this include the UDF filesystem used by DVD-ROM's,
> and Digital Unix's Advanced Filesystem.
What "feels wrong" about this to me is that all fs implementations are
then required to implement multiple device spanning, or they can't be
used on spanning partitions at all.
Well, for filesystems that don't implement multiple device spanning,
they can use MD (or LVM) today. But there are advantages to getting the
filesystem involved; the filesystem can more efficiently handle
placement issues, and it can more easily handle evacuating a disk used
in the middle of a logical volume.
It's certainly simpler and more intellectually satisfying to say that
all of these issues should be completely handled in an "lower layer',
but STREAMS made the same argument about networking, as you may recall.
Sometimes the most satisfying abstraction boundaries don't result in the
most efficient and performant implementation.
- Ted
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