linux-kernel - Re: clustered MD

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Message-ID: <20150609203056.GB17536@redhat.com>
Date:	Tue, 9 Jun 2015 15:30:56 -0500
From:	David Teigland <teigland@...hat.com>
To:	Goldwyn Rodrigues <rgoldwyn@...e.com>
Cc:	linux-kernel@...r.kernel.org, NeilBrown <neilb@...e.de>
Subject: Re: clustered MD

On Tue, Jun 09, 2015 at 03:08:11PM -0500, Goldwyn Rodrigues wrote:
> Hi David,
> 
> On 06/09/2015 02:45 PM, David Teigland wrote:
> >On Tue, Jun 09, 2015 at 02:26:25PM -0500, Goldwyn Rodrigues wrote:
> >>On 06/09/2015 01:22 PM, David Teigland wrote:
> >>>I've just noticed the existence of clustered MD for the first time.
> >>>It is a major new user of the dlm, and I have some doubts about it.
> >>>When did this appear on the mailing list for review?
> >>
> >>It first appeared in December, 2014 on the RAID mailing list.
> >>http://marc.info/?l=linux-raid&m=141891941330336&w=2
> >
> >I don't read that mailing list.  Searching my archives of linux-kernel, it
> >has never been mentioned.  I can't even find an email for the md pull
> >request that included it.
> 
> Is this what you are looking for?
> http://marc.info/?l=linux-kernel&m=142976971510061&w=2

Yes, I guess gmail lost it, or put it in spam.

> >- "experimental" code for managing md/raid1 across a cluster using
> >      DLM.  Code is not ready for general use and triggers a WARNING if
> >      used.  However it is looking good and mostly done and having in
> >      mainline will help co-ordinate development.
> >
> >That falls far short of the bar for adding it to the kernel.  It not only
> >needs to work, it needs to be reviewed and justified, usually by showing
> 
> Why do you say it does not work?

It's just my abbreviation of that summary paragraph.

> It did go through it's round of reviews on the RAID mailing list. I
> understand that you missed it because you are not subscribed to the raid
> mailing list.

I will look for that.

> >some real world utility to warrant the potential maintenance effort.
> 
> We do have a valid real world utility. It is to provide
> high-availability of RAID1 storage  over the cluster. The
> distributed locking is required only during cases of error and
> superblock updates and is not required during normal operations,
> which makes it fast enough for usual case scenarios.

That's the theory, how much evidence do you have of that in practice?

> What are the doubts you have about it?

Before I begin reviewing the implementation, I'd like to better understand
what it is about the existing raid1 that doesn't work correctly for what
you'd like to do with it, i.e. I don't know what the problem is.

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