Re: [mdadm PATCH] Create: tell udev md device is not ready when first created.

[Jes Sorensen <jes.sorensen@gmail.com>]

On 04/28/2017 05:28 AM, Peter Rajnoha wrote:
> On 04/28/2017 07:05 AM, NeilBrown wrote:
>>
>> When an array is created the content is not initialized,
>> so it could have remnants of an old filesystem or md array
>> etc on it.
>> udev will see this and might try to activate it, which is almost
>> certainly not what is wanted.
>>
>> So create a mechanism for mdadm to communicate with udev to tell
>> it that the device isn't ready.  This mechanism is the existance
>> of a file /run/mdadm/created-mdXXX where mdXXX is the md device name.
>>
>> When creating an array, mdadm will create the file.
>> A new udev rule file, 01-md-raid-creating.rules, will detect the
>> precense of thst file and set ENV{SYSTEMD_READY}="0".
>> This is fairly uniformly used to suppress actions based on the
>> contents of the device.
>
> The scans in udev are primarily directed by blkid call which detects the
> signatures and based on this information various other udev rules fire.
>
> The blkid as well as wipefs uses common libblkid library to detect these
> signatures - is mdadm going to use libblkid to wipe the signatures on MD
> device initialization or is it relying on external tools to do this? How
> is mdadm actually initializing/wiping the newly created MD device?

mdadm doesn't wipe data and it isn't supposed to. Being able to create 
an array from drives with existing data is a feature.

It is the responsibility of the system administrator to handle drives 
with existing data, in the same way the administrator is expected to 
handle insertion of USB drives or external drives being powered on.

Jes