[PATCH v2 0/3] Bitmap percentage flushing

[Jonathan Derrick <jonathan.derrick@linux.dev>]

This introduces a percentage-flushing mechanism that works in-tandem to the
mdadm delay timer. The percentage argument is based on the number of chunks
dirty (rather than percentage), due to large drives requiring smaller and
smaller percentages (eg, 32TB drives-> 1% is 320GB).

This set hopes to provide a way to make the bitmap flushing more consistent. It
was observed that a synchronous, random write qd1 workload, could make bitmap
writes easily become almost half of the I/O. And in similar workloads with
different timing, it was several minutes between bitmap updates. This is too
inconsistent to be reliable.

This first and second patches adds the flush_threshold parameter. The default
value of 0 defines the default behavior: unplugging immediately just as before.
With a flush-threshold value of 1, it becomes more consistent and paranoid,
flushing on nearly every I/O, leading to a 40% or greater situation. From
there, the flush_threshold can be defined higher for those situations where
power loss is rare and full resync can be tolerated.

The third patch converts the daemon worker to an actual timer. This makes it
more consistent and removes some ugly code.

Jonathan Derrick (3):
  md/bitmap: Add chunk-threshold unplugging
  md/bitmap: Add sysfs interface for flush threshold
  md/bitmap: Convert daemon_work to proper timer

 Documentation/admin-guide/md.rst |  5 ++
 drivers/md/md-bitmap.c           | 98 +++++++++++++++++++++++++-------
 drivers/md/md-bitmap.h           |  4 +-
 drivers/md/md.c                  |  9 ++-
 drivers/md/md.h                  |  2 +
 5 files changed, 93 insertions(+), 25 deletions(-)

-- 
2.31.1