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

[Christoph Hellwig]

On Tue, Feb 11, 2025 at 01:25:59PM -0500, Alyssa Rosenzweig wrote:
> From: Jens Axboe <axboe at kernel.dk>
> 
> 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 :)