Re: mdadm: Patch to restrict --size when shrinking unless forced

[NeilBrown <neilb@suse.com>]

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On Mon, Oct 09 2017, Phil Turmel wrote:

> On 10/09/2017 12:10 AM, NeilBrown wrote:
>
>> If there is some action that mdadm can currently be told to perform, and
>> when it tries to perform that action it corrupts the array, then
>> it is certainly appropriate to teach mdadm not to perform that action.
>> It shouldn't even perform that action with --force.   I agree that
>> changing mdadm like this is complementary to changing the kernel.  Both
>> are useful.
>
> A certain amount of the trouble with all of this is the english meaning
> of "grow" doesn't really match what mdadm allows.
>
> Might it be reasonable to reject "--grow" operations that reduce the
> final array size, and introduce the complementary "--reduce" operation
> that rejects array size increases?

While there is a lot to like about this approach, one problem is that
some "grow" operations do not change the size. They might, e.g., just change
the chunksize.

I guess you could have --grow --reduce --reshape.

I wouldn't object to such a change.

Thanks,
NeilBrown

>
> Both operations would share the current code, just apply a different
> sanity check before proceeding.
>
> mdadm would then at least not violate the rule of least surprise.
>
> Phil

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