linux-kernel - Re: clustered MD

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Message-ID: <557747AB.7080706@suse.com>
Date:	Tue, 09 Jun 2015 15:08:11 -0500
From:	Goldwyn Rodrigues <rgoldwyn@...e.com>
To:	David Teigland <teigland@...hat.com>
CC:	linux-kernel@...r.kernel.org, NeilBrown <neilb@...e.de>
Subject: Re: clustered MD

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

>
> The merge commit states:
>
> - "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 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.

> 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.

What are the doubts you have about it?

-- 
Goldwyn
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