Re: clustered MD

From: Goldwyn Rodrigues
Date: Tue Jun 09 2015 - 16:14:24 EST


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
some real world utility to warrant the potential maintenance effort.



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.

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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