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    SubjectRe: [PATCH 000 of 5] md: Introduction
    On Thu, Jan 19, 2006 at 09:17:12PM -0500, Phillip Susi wrote:
    > Neil Brown wrote:
    >
    > >Maybe the problem here is thinking of md and dm as different things.
    > >Try just not thinking of them at all.
    > >Think about it like this:
    > > The linux kernel support lvm
    > > The linux kernel support multipath
    > > The linux kernel support snapshots
    > > The linux kernel support raid0
    > > The linux kernel support raid1
    > > The linux kernel support raid5
    > >
    > >Use the bits that you want, and not the bits that you don't.
    > >
    > >dm and md are just two different interface styles to various bits of
    > >this. Neither is clearly better than the other, partly because
    > >different people have different tastes.
    > >
    > >Maybe what you really want is for all of these functions to be managed
    > >under the one umbrella application. I think that is was EVMS tried to
    > >do.
    > >
    > >
    > >
    >
    > I am under the impression that dm is simpler/cleaner than md. That
    > impression very well may be wrong, but if it is simpler, then that's a
    > good thing.
    >
    >
    > >One big selling point that 'dm' has is 'dmraid' - a tool that allows
    > >you to use a lot of 'fakeraid' cards. People would like dmraid to
    > >work with raid5 as well, and that is a good goal.
    > >
    > >
    >
    > AFAIK, the hardware fakeraid solutions on the market don't support raid5
    > anyhow ( at least mine doesn't ), so dmraid won't either.

    Well, some do (eg, Nvidia).

    >
    > >However it doesn't mean that dm needs to get it's own raid5
    > >implementation or that md/raid5 needs to be merged with dm.
    > >It can be achieved by giving md/raid5 the right interfaces so that
    > >metadata can be managed from userspace (and I am nearly there).
    > >Then 'dmraid' (or a similar tool) can use 'dm' interfaces for some
    > >raid levels and 'md' interfaces for others.
    > >
    >
    > Having two sets of interfaces and retrofiting a new interface onto a
    > system that wasn't designed for it seems likely to bloat the kernel with
    > complex code. I don't really know if that is the case because I have
    > not studied the code, but that's the impression I get, and if it's
    > right, then I'd say it is better to stick with dm rather than retrofit
    > md. In either case, it seems overly complex to have to deal with both.

    I agree, but dm will need to mature before it'll be able to substitute md.

    >
    >
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    --

    Regards,
    Heinz -- The LVM Guy --

    *** Software bugs are stupid.
    Nevertheless it needs not so stupid people to solve them ***

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    Heinz Mauelshagen Red Hat GmbH
    Consulting Development Engineer Am Sonnenhang 11
    Cluster and Storage Development 56242 Marienrachdorf
    Germany
    Mauelshagen@RedHat.com +49 2626 141200
    FAX 924446
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