linux-kernel - Re: [GIT PULL] DRBD for 2.6.32

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Date:	Wed, 23 Sep 2009 20:27:51 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	kyle@...fetthome.net
Cc:	fujita.tomonori@....ntt.co.jp, lars.ellenberg@...bit.com,
	arjan@...radead.org, lmb@...e.de, jens.axboe@...cle.com,
	neilb@...e.de, hch@...radead.org, James.Bottomley@...e.de,
	linux-kernel@...r.kernel.org, drbd-dev@...ts.linbit.com,
	akpm@...ux-foundation.org, bart.vanassche@...il.com,
	davej@...hat.com, gregkh@...e.de, kosaki.motohiro@...fujitsu.com,
	torvalds@...ux-foundation.org, nab@...ux-iscsi.org,
	knikanth@...e.de, philipp.reisner@...bit.com, sam@...nborg.org,
	Mauelshagen@...hat.com
Subject: Re: [GIT PULL] DRBD for 2.6.32

On Mon, 21 Sep 2009 20:51:32 -0400
Kyle Moffett <kyle@...fetthome.net> wrote:

> On Mon, Sep 21, 2009 at 18:27, FUJITA Tomonori
> <fujita.tomonori@....ntt.co.jp> wrote:
> > On Mon, 21 Sep 2009 18:53:21 +0200 Lars Ellenberg <lars.ellenberg@...bit.com> wrote:
> >> That's not what I meant, of course that is and needs to be stable.
> >> Sorry, I exagerated to make a point.
> >>
> >> Point was:
> >> mdadm configured md.
> >> dmsetup configured dm.
> >> drbdsetup configure drbd.
> >>
> >> If and when "something" is done to "unify" things on the implementation
> >> level, it is likely to also unify the "kernel<->userspace" configuration
> >> interface.
> >>
> >> If it happens, once that happens, that _will_ be an ABI break.
> >
> > You misunderstand the raid unification.
> >
> > We will not unify the kernel<->userspace configuration interface
> > because we can't break the kernel<->userspace ABI.
> >
> > We plan to unify the multiple device frameworks, but the unified
> > framework must support the all existing ABIs.
> >
> > So adding another 'drbd' ABI hurts us.
> 
> One major issue for me personally (and I don't think its been mentioned enough):
> 
> There is a *VAST* existing user-base for DRBD.  Basically every vendor
> builds the modules for their kernels, ships the userspace tools, etc.
> *Regardless* of when or how it gets merged, the existing user-base
> will need kernel support for the existing tools.

I don't think that the user base can be a reason for mainline
inclusion.

IMHO, vendors should use their resource to push an out-of-tree thing
into mainline instead of taking care of it with their own
kernels. Finally, device-mapper people are trying to push the similar
feature. I think that the history taught us that people who have used
out-of-tree stuff eventually move in the mainline alternative.


> To put it another way:  Would you really keep a stable SCSI raid
> driver for existing hardware out of mainline by claiming they need to
> write a new raid-management abstraction first?  If not, then why the
> pushback on DRBD?

Yeah, we should have done that. It's too late though.

Anyway, I don't think that your example is fair; we need a driver
for scsi hardware but we have an alternative to drbd. 
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