[PATCH v4 0/3] btrfs: read_policy types latency, device and round-robin

[Anand Jain <anand.jain@oracle.com>]

v4:
Add rb from Josef in patch 1 and 3.
In patch 1/3, use fs_info instead of device->fs_devices->fs_info.
Drop round-robin policy because my workload (fio random) shows no performance
 gains due to fewer merges at the block layer.

v3:
The block layer commit 0d02129e76ed (block: merge struct block_device and
struct hd_struct) has changed the first argument in the function
part_stat_read_all() in 5.11-rc1. So trickle down its changes in the patch 1/4.

v2:
Fixes as per review comments, as in the individual patches.

rfc->v1:
Drop the tracing patch.
Drop the factor associated with the inflight commands (because there
were too many unnecessary switches).
Few C styles fix.

-----

This patchset adds read policy types latency, device, and round-robin, for the
mirrored raid profiles such as raid1, raid1c3, raid1c4, and raid10. The default
read policy remains as PID, as of now.

Read policy types:
Latency:

Latency policy routes the read IO based on the historical average
wait time experienced by the read IOs on the individual device.

Device:

With the device policy along with the read_preferred flag, you can
set the device for reading manually. Useful to test mirrors in a
deterministic way and helps advance system administrations.

Round-robin (RFC patch):

Alternates striped device in a round-robin loop for reading. To achieve
this first we put the stripes in an array, sort it by devid and pick the
next device.

Test scripts:
=============

I have included a few scripts which were useful for testing.

-------------------8<--------------------------------
Set latency policy on the btrfs mounted at /mnt

Usage example:
  $ readpolicyset /mnt latency

Anand Jain (3):
  btrfs: add read_policy latency
  btrfs: introduce new device-state read_preferred
  btrfs: introduce new read_policy device

 fs/btrfs/sysfs.c   | 57 ++++++++++++++++++++++++++++++++++++++++++-
 fs/btrfs/volumes.c | 60 ++++++++++++++++++++++++++++++++++++++++++++++
 fs/btrfs/volumes.h |  5 ++++
 3 files changed, 121 insertions(+), 1 deletion(-)

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2.28.0