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Re: [Libguestfs] [PATCH] New API: mdadm-stop for stopping MD devices.

On 23/11/11 17:11, Matthew Booth wrote:
On 23/11/11 16:22, Richard W.M. Jones wrote:
On Wed, Nov 23, 2011 at 03:02:19PM +0000, Matthew Booth wrote:
On 23/11/11 14:43, Wanlong Gao wrote:
This API is used to stop a md device.
When we want to move a device to another md array, we should
stop the md device which contained this device first.

I'm not convinced we should extend the md api in libguestfs to
active manipulation. I wasn't convinced that mdadm-create was
worthwhile either, but it made a test case ever so slightly tidier.
To me, MD devices are the domain of physical machines. We need to be
able to *read* them (for P2V apart from anything else), but I think
manipulation is opening an unnecessary can of worms.

But is there any penalty to carrying this?

I can think of 2:

1. Maintenance: I know you think this is free, but some day mdadm --stop
might change behaviour, and then the api will break.

2. A false sense of expectation: Having an incomplete md api can only
lead to frustration. Implementing a complete md api would require a
large amount of effort for something which we don't have a convincing
use case for (i.e. nobody's ever going to use it anyway).

There's another good one, actually: Testing. mdadm-create has already created a testing problem, because test-mdadm.sh fails mysteriously on my machine, but didn't before and doesn't for you or Wanlong. Especially for something like MD, which has a complex subsystem underlying it which we don't fully understand in our team, adding and adequately testing a new API is an expensive operation.

Matthew Booth, RHCA, RHCSS
Red Hat Engineering, Virtualisation Team

GPG ID:  D33C3490
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