I've been looking at what's the best way to make sure everything gets cleanly flushed out to disk on a powerdown. Right now in __orderly_poweroff(), we call emergency_sync() which kicks a workqueue to flush all file systems and block devices --- and then we immediately power down the system, before the scheduler even has a chance to schedule the workqueue thread. Hopefully userspace has the unmounted all file systems, which will has implicitly issued a cache flush command, but if we have a userspace program writing to a block device directly, currently there's nothing to make sure things will get flushed out to the device. Beyond that, though, I'm interested in figuring out how to make sure that all SCSI devices will receive (and acknowledge) SHUTDOWN command so that the disks can be spun down and heads retracted to a safe landing zone before we power down the system. It appears the best way to do this is to call sd_shutdown(), since we don't seem to have a high-level "shutdown" concept recognized in the block layer (the way we currently, have, say support for "discard"). So the question is, what's the best way to architect something like this. I could implement a hacky interator loop in the SCSI subsystem, and call it directly from __orderly_poweroff in kernel/reboot.c. But I'm pretty sure that would never get accepted upstream, and so it would remain a Google data center hack. What do people think would be the best way of implementing something that would be upstream acceptable? Thanks, - Ted
