[PATCH 3/3] apple-nvme: defer cache flushes by a specified amount

[Alyssa Rosenzweig]

> > Cache flushes on the M1 nvme are really slow, taking 17-18 msec to
> > complete. This can slow down workloads considerably, pure random writes
> > end up being bound by the flush latency and hence run at 55-60 IOPS.
> > 
> > Add a deferred flush work around to provide better performance, at a
> > minimal risk. By default, flushes are delayed at most 1 second, but this
> > is configurable.
> > 
> > With this work-around, a pure random write workload runs at ~12K IOPS
> > rather than 56 IOPS.
> 
> Just as last time this really is not a driver feature.  Cache flushes
> are slow on consumer hardware, it's just apple is worse than usual.
> Breaking file system transactional guarantee by ignoring data integrity
> command in the driver is a no-go.
> 
> If we want to allow an opt-in policy for those whole feel adventurous,
> it belongs into the core flush state machine.  Fortunately the patch
> author seems qualified to touch that :)

Fair enough. I didn't realize this patch was previously discussed, my
apologies. I'll drop this change in v2, and hopefully somebody is
inspired later to do that 'adventure'.