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[dm-devel] MD and DM (was: Re: ANNOUNCE: mdadm 3.0 - A tool for managing Soft RAID under Linux)



On Tue, Jun 2, 2009 at 6:58 PM, Dan Williams <dan j williams intel com> wrote:
> On Tue, Jun 2, 2009 at 1:11 PM, Jeff Garzik <jeff garzik org> wrote:
>> Neil Brown wrote:
>>>
>>> I am pleased to (finally) announce the availability of
>>>   mdadm version 3.0
>>>
>>> It is available at the usual places:
>>>   countrycode=xx.
>>>   http://www.${countrycode}kernel.org/pub/linux/utils/raid/mdadm/
>>> and via git at
>>>   git://neil.brown.name/mdadm
>>>   http://neil.brown.name/git?p=mdadm
>>>
>>>
>>> This is a major new version and as such should be treated with some
>>> caution.  However it has seen substantial testing and is considerred
>>> to be ready for wide use.
>>>
>>>
>>> The significant change which justifies the new major version number is
>>> that mdadm can now handle metadata updates entirely in userspace.
>>> This allows mdadm to support metadata formats that the kernel knows
>>> nothing about.
>>>
>>> Currently two such metadata formats are supported:
>>>  - DDF  - The SNIA standard format
>>>  - Intel Matrix - The metadata used by recent Intel ICH controlers.
>>
>> This seems pretty awful from a support standpoint:  dmraid has been the sole
>> provider of support for vendor-proprietary up until this point.
>
> This bares similarities with the early difficulties of selecting
> between ide and libata.
>
>> Now Linux users -- and distro installers -- must choose between software
>> RAID stack "MD" and software RAID stack "DM".  That choice is made _not_
>> based on features, but on knowing the underlying RAID metadata format that
>> is required, and what features you need out of it.
>>
>> dmraid already supports
>>        - Intel RAID format, touched by Intel as recently as 2007
>>        - DDF, the SNIA standard format
>>
>> This obviously generates some relevant questions...
>>
>> 1) Why?  This obviously duplicates existing effort and code.  The only
>> compelling reason I see is RAID5 support, which DM lacks IIRC -- but the
>> huge issue of user support and duplicated code remains.
>
> The MD raid5 code has been upstream since forever and already has
> features like online capacity expansion.  There is also
> infrastructure, upstream, for online raid level migration.
>
>> 2) Adding container-like handling obviously moves MD in the direction of DM.
>>  Does that imply someone will be looking at integrating the two codebases,
>> or will this begin to implement features also found in DM's codebase?
>
> I made a proof-of-concept investigation of what it would take to
> activate all dmraid arrays (any metadata format, any raid level) with
> MD.  The result, dm2md [1], did not stimulate much in the way of
> conversation.
>
> A pluggable architecture for a write-intent log seems to be the only
> piece that does not have a current equivalent in MD.  However, the
> 'bitmap' infrastructure covers most needs.  I think unifying on a
> write-intent logging infrastructure is a good place to start working
> together.
>
>> 3) What is the status of distro integration efforts?  I wager the distro
>> installer guys will grumble at having to choose among duplicated RAID code
>> and formats.
>
> There has been some grumbling, but the benefits of using one
> linux-raid infrastructure for md-metadata and vendor metadata is
> appealing.  mdadm-3.0 also makes a serious effort to be more agreeable
> with udev and incremental discovery.  So hopefully this makes mdadm
> easier to handle in the installer.
>
>> 4) What is the plan for handling existing Intel RAID users (e.g. dmraid +
>> Intel RAID)?  Has Intel been contacted about dmraid issues?  What does Intel
>> think about this lovely user confusion shoved into their laps?
>
> The confusion was the other way round.  We were faced with how to
> achieve long term feature parity of our raid solution across OS's and
> the community presented us with two directions DM and MD.  The
> decision was made to support and maintain dmraid for existing
> deployments while basing future development on extending the MD stack,
> because it gave some feature advantages out of the gate.  So, there is
> support for both and new development will focus on MD.
>
>> 5) Have the dmraid maintainer and DM folks been queried, given that you are
>> duplicating their functionality via Intel and DDF RAID formats? What was
>> their response, what issues were raised and resolved?
>
> There have been interludes, but not much in the way of discussion.
> Hopefully, this will be a starting point.
>
> Thanks,
> Dan
>
> [1] http://marc.info/?l=linux-raid&m=123300614013042&w=2

dm-devel should be cc'd to get a dialog going with the DM team.


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