Re: [PATCH 03/24] nvme: let set_capacity_revalidate_and_notify update the bdev size

[Hannes Reinecke <hare@suse.de>]

On 11/10/20 12:28 AM, Sagi Grimberg wrote:
> 
>> [ .. ]
>>>> Originally nvme multipath would update/change the size of the multipath
>>>> device according to the underlying path devices.
>>>> With this patch the size of the multipath device will _not_ change 
>>>> if there
>>>> is a change on the underlying devices.
>>>
>>> Yes, it will.  Take a close look at nvme_update_disk_info and how it is
>>> called.
>>>
>> Okay, then: What would be the correct way of handling a size update 
>> for NVMe multipath?
>> Assuming we're getting an AEN for each path signalling the size change
>> (or a controller reset leading to a size change).
>> So if we're updating the size of the multipath device together with 
>> the path device at the first AEN/reset we'll end up with the other 
>> paths having a different size than the multipath device (and the path 
>> we've just been updating).
>> - Do we care, or cross fingers and hope for the best?
>> - Shouldn't we detect the case where we won't get a size update for 
>> the other paths, or, indeed, we have a genuine device size mismatch 
>> due to a misconfiguration on the target?
>>
>> IE shouldn't we have a flag 'size update pending' for the other 
>> paths,, to take them out ouf use temporarily until the other 
>> AENs/resets have been processed?
> 
> the mpath device will take the minimum size from all the paths, that is
> what blk_stack_limits does. When the AEN for all the paths will arrive
> the mpath size will update.
> 
But that's precisely my point; there won't be an AEN for _all_ paths, 
but rather one AEN per path. Which will be processed separately, leading 
to the issue described above.

> Not sure how this is different than what we have today...

Oh, that is a problem even today.
So we should probably move it to a different thread...

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
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