[dm-devel] [PATCH 1/2] md/raid0: Introduce new array state 'broken' for raid0

[Guilherme G. Piccoli]

On 30/07/2019 03:20, Bob Liu wrote:
> [...]
>> + * broken
>> + *     RAID0-only: same as clean, but array is missing a member.
>> + *     It's useful because RAID0 mounted-arrays aren't stopped
>> + *     when a member is gone, so this state will at least alert
>> + *     the user that something is wrong.
> 
> 
> Curious why only raid0 has this issue? 
> 
> Thanks, -Bob

Hi Bob, I understand that all other levels have fault-tolerance logic,
while raid0 is just a "bypass" driver that selects the correct
underlying device to send the BIO and blindly sends it. It's known to be
a performance-only /lightweight solution whereas the other levels aim to
be reliable.

I've quickly tested raid5 and rai10, and see messages like this on
kernel log when removing a device (in raid5):

[35.764975] md/raid:md0: Disk failure on nvme1n1, disabling device.
md/raid:md0: Operation continuing on 1 devices.

The message seen in raid10 is basically the same. As a (cheap)
comparison of the complexity among levels, look that:

<...>/linux-mainline/drivers/md# cat raid5* | wc -l
14191

<...>/linux-mainline/drivers/md# cat raid10* | wc -l
5135

<...>/linux-mainline/drivers/md# cat raid0* | wc -l
820

Cheers,


Guilherme