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SubjectRe: Volume Managers in Linux

James Fidell writes:
> 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.

You always get simple concatenated partition support from MD.
Filesystem support is only needed if you want to go beyond that.

> Conceptually it seems simpler to have the virtual layer which understands
> how to span multiple partitions, but which looks like a block device from
> the "user" view, thus allowing any filesystem type to be used upon it, be
> that ext2, reiserfs, ufs or something even better that we haven't even

For UFS, you'd want to be disk-level compatible with whatever Solaris
and FreeBSD have. Hans Reiser wants to span Reiserfs across a network,
but that won't work too well with a dumb LVM system. NTFS has it's own
system of course, and couldn't use a Linux-specific LVM. That leaves
only ext2 and a collection of odd filesystems. Since ext2 would have
to be aware of the system anyway, the extra layer can be eliminated.

It has taken a long time to generate enough desire to advance the
way Linux deals with partitions. The LVM is a small step that will
drain away some of the desire to help code a device-spanning filesystem.
Minor issues of performance and awkwardness might not be enough to
get people hacking on something better for several years. I can see
several years of "just unmount, resize the LVM, resize ext2, and
mount the filesystem again". That's offline, not online.

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