Re: Multi-Actuator SAS HDD First Look

[Hannes Reinecke <hare@suse.de>]

On Fri, 23 Mar 2018 08:57:12 -0600
Tim Walker <tim.t.walker@seagate.com> wrote:

> Seagate announced their split actuator SAS drive, which will probably
> require some kernel changes for full support. It's targeted at cloud
> provider JBODs and RAID.
> 
> Here are some of the drive's architectural points. Since the two LUNs
> share many common components (e.g. spindle) Seagate allocated some
> SCSI operations to be LUN specific and some to affect the entire
> device, that is, both LUNs.
> 
> 1. Two LUNs, 0 & 1, each with independent lba space, and each
> connected to an independent read channel, actuator, and set of heads.
> 2. Each actuator addresses 1/2 of the media - no media is shared
> across the actuators. They seek independently.
> 3. One World Wide Name (WWN) is assigned to the port for device
> address. Each Logical Unit has a separate World Wide Name for
> identification in VPD page.
> 4. 128 deep command queue, shared across both LUNs
> 5. Each LUN can pull commands from the queue independently, so they
> can implement their own sorting and optimization.
> 6. Ordered tag attribute causes the command to be ordered across both
> Logical Units
> 7. Head of Queue attribute causes the command to be ordered with
> respect to a single Logical Unit
> 8. Mode pages are device-based (shared across both Logical Units)
> 9. Log pages are device-based.
> 10. Inquiry VPD pages (with minor exceptions) are device based.
> 11. Device health features (SMART, etc) are device based
> 
> Seagate wants the multi-actuator design to integrate into the stack as
> painlessly as possible.The interface design is still in the early
> stages, so I am gathering requirements and recommendations, and also
> providing any information necessary to help scope integrating a
> multi-LUN device into the MQ stack. So, I am soliciting any pertinent
> feedback including:
> 
> 1. Painful incompatibilities between the Seagate proposal and current
> MQ architecture
> 2. Linux changes needed
> 3. Drive interface changes needed
> 4. Anything else I may have overlooked
> 
So far it looks okay; just make sure to have VPD page 0x83
entries properly associated.
To all intents and purposes these devices seem to look like 'normal'
devices with two LUNs; nothing special with that.
Real question would be in which areas those devices differentiate from
the two indepdendent LUN scenario.

There might be issues with per-device informations like SMART etc;
ideally they are available from _both_ LUNs.
Otherwise they'll show up as blank from one LUN, causing consternation
with any management software.

Cheers,

Hannes