Re: [PATCH 1/2] scsi: sd: set ready_to_power_off for scsi disk

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On Thu, 2012-09-13 at 16:49 +0800, Aaron Lu wrote:
> On 09/13/2012 04:37 PM, James Bottomley wrote:
> > On Thu, 2012-09-13 at 16:23 +0800, Aaron Lu wrote:
> >> On 09/13/2012 04:14 PM, James Bottomley wrote:
> >>> On Thu, 2012-09-13 at 15:40 +0800, Aaron Lu wrote:
> >>>> The ready_to_power_off flag is used to give indication to ATA layer
> >>>> if this device's power can be removed when runtime suspended.
> >>>>
> >>>> This flag is determined by individual SCSI driver like sr, sd.
> >>>>
> >>>> This flag is introduced to support zero power ODD. When ODD
> >>>> is runtime suspended, it may not be OK to remove its power.
> >>>>
> >>>> But for disk, it is always OK to be powered off, so set this flag.
> >>>
> >>> It is? I may have missed this, but where do you flush the cache of write
> >>> back cache devices you're about to power off?
> >>
> >> I suppose that is handled in sd_suspend callback, the power off happens
> >> after a device is runtime suspended.
> > 
> > Well that would mean something is wrong somewhere:  For runtime power
> > management using idle timers and forced standby, there's no need to
> 
> The current mechanism for scsi disk runtime pm is based on open/close.
> If there is some process opened this block device, it will be in active
> state; only when all opened session exited, it will enter runtime
> suspend state.

A mounted disk is open for the period of the mount.  I thought the use
case for runtime PM was the laptop one but most laptops have a single
device to use as root, so if you never use runtime PM on an open device,
you never use it on 99% of our target systems ... doesn't that make the
feature a bit useless?

> > flush the cache (if the drive goes into standby on its own as a result
> > of an idle timeout, the cache will never flush).  The cache needs to
> > flush before we power off the device: that's before the system goes into
> > S3, or now before you power it off at runtime.  Flushing the cache on
> > runtime transitions to standby will likely cause performance problems
> > since that happens quite often.
> 
> As explained above, it didn't happen that often, especially for user who
> has only one disk, the disk will be mounted, which makes it never be
> able to enter runtime suspend state.

So what's the target audience for the feature.  If it isn't laptops or
standard desktops, is it the enterprise?

James

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