[PATCH v4 0/1] nvme: queue-depth multipath iopolicy

[John Meneghini]

Sorry this took so long. I've fixed everything up and rebased this
(again) onto nvme-6.10 so as to pick up Keith's 'io accounting on
failover fix'.  A new build is being tested again for good measure.

I'll send a Tested-by: John Meneghini once the tests are done.

Changes since V3:

Addresssed all review comments, fixed the commit log, and moved
nr_counter initialization from nvme_mpath_init_ctlr() to
nvme_mpath_init_identify().

Changes since V2:

Add the NVME_MPATH_CNT_ACTIVE flag to eliminate a READ_ONCE in the completion path
and increment/decrement the active_nr count on all mpath IOs - including
passthru commands.

Send a pr_notice when ever the iopolicy on a subsystem is changed. This is
important for support reasons. It is fully expected that users will be changing
the iopolicy with active IO in progress.

Squashed everything and rebased to nvme-v6.10

Changes since V1:

I'm re-issuing Ewan's queue-depth patches in preparation for LSFMM

These patches were first show at ALPSS 2023 where I shared the following
graphs which measure the IO distribution across 4 active-optimized
controllers using the round-robin verses queue-depth iopolicy.

 https://people.redhat.com/jmeneghi/ALPSS_2023/NVMe_QD_Multipathing.pdf

Since that time we have continued testing these patches with a number of
different nvme-of storage arrays and test bed configurations, and I've codified
the tests and methods we use to measure IO distribution

All of my test results, together with the scripts I used to generate these
graphs, are available at:

 https://github.com/johnmeneghini/iopolicy

Please use the scripts in this repository to do your own testing.

These patches are based on nvme-v6.9



Ewan D. Milne (1):
  nvme: multipath: Implemented new iopolicy "queue-depth"

 drivers/nvme/host/core.c      |  2 +-
 drivers/nvme/host/multipath.c | 91 +++++++++++++++++++++++++++++++++--
 drivers/nvme/host/nvme.h      |  8 +++
 3 files changed, 96 insertions(+), 5 deletions(-)

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2.39.3