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