[LSF/MM/BPF TOPIC] dispersed namespaces revisited

[Hannes Reinecke]

Hi all,

there had been quite some discussion on various venues about dispersed 
namespaces on NVMe and missing linux support.
Especially since it looks as if the original specification will not be 
implemented, yet vendors do view it as a crucial use-case.
Which is already supported on other protocols like SCSI just fine, I 
might add. Even on Linux.

So I would like to have a discussion on where we stand, what the 
proposals are, and what we can do from the linux side to support the use 
case.

To add a bit of background:
Dispersed namespaces have been defined to support live migration of data 
from one subsystem to another. General idea is that the same namespace 
(as identified by the namespace identifier) might show up on different 
subsystems.
This is already working on SCSI, as dm multipathing will just look as 
the VPD page identifcation and arrange devices based on that.
For NVMe with native multipathing this currently does not work, as
a) we're identifying namespaces with the numerical NSID
and
b) namespaces are attached to the subsystem, and can only be assembled 
within that subsystem.

Sure we can always switch back to device-mapper multipathing, but I 
don't think that's a direction we want to go.
(I certainly don't.)

This discussion will be on how do we go from here; changing the spec 
and/or the implementation is on the table.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare at suse.de                              +49 911 74053 688
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