[PATCH RFC 0/6] Add roundrobin raid1 read policy

[Michal Rostecki]

From: Michal Rostecki <mrostecki@xxxxxxxx>

This patch series adds a new raid1 read policy - roundrobin. For each
request, it selects the mirror which has lower load than queue depth.
Load is defined  as the number of inflight requests + a penalty value
(if the scheduled request is not local to the last processed request for
a rotational disk).

The series consists of preparational changes which add necessary
information to the btrfs_device struct and the change with the policy.

This policy was tested with fio and compared with the default `pid`
policy.

The singlethreaded test has the following parameters:

  [global]
  name=btrfs-raid1-seqread
  filename=btrfs-raid1-seqread
  rw=read
  bs=64k
  direct=0
  numjobs=1
  time_based=0

  [file1]
  size=10G
  ioengine=libaio

and shows the following results:

- raid1c3 with 3 HDDs:
  3 x Segate Barracuda ST2000DM008 (2TB)
  * pid policy
    READ: bw=217MiB/s (228MB/s), 217MiB/s-217MiB/s (228MB/s-228MB/s),
    io=10.0GiB (10.7GB), run=47082-47082msec
  * roundrobin policy
    READ: bw=409MiB/s (429MB/s), 409MiB/s-409MiB/s (429MB/s-429MB/s),
    io=10.0GiB (10.7GB), run=25028-25028mse
- raid1c3 with 2 HDDs and 1 SSD:
  2 x Segate Barracuda ST2000DM008 (2TB)
  1 x Crucial CT256M550SSD1 (256GB)
  * pid policy (the worst case when only HDDs were chosen)
    READ: bw=220MiB/s (231MB/s), 220MiB/s-220MiB/s (231MB/s-231MB/s),
    io=10.0GiB (10.7GB), run=46577-46577mse
  * pid policy (the best case when SSD was used as well)
    READ: bw=513MiB/s (538MB/s), 513MiB/s-513MiB/s (538MB/s-538MB/s),
    io=10.0GiB (10.7GB), run=19954-19954msec
  * roundrobin (there are no noticeable differences when testing multiple
    times)
    READ: bw=541MiB/s (567MB/s), 541MiB/s-541MiB/s (567MB/s-567MB/s),
    io=10.0GiB (10.7GB), run=18933-18933msec

The multithreaded test has the following parameters:

  [global]
  name=btrfs-raid1-seqread
  filename=btrfs-raid1-seqread
  rw=read
  bs=64k
  direct=0
  numjobs=8
  time_based=0

  [file1]
  size=10G
  ioengine=libaio

and shows the following results:

- raid1c3 with 3 HDDs: 3 x Segate Barracuda ST2000DM008 (2TB)
  3 x Segate Barracuda ST2000DM008 (2TB)
  * pid policy
    READ: bw=1569MiB/s (1645MB/s), 196MiB/s-196MiB/s (206MB/s-206MB/s),
    io=80.0GiB (85.9GB), run=52210-52211msec
  * roundrobin
    READ: bw=1733MiB/s (1817MB/s), 217MiB/s-217MiB/s (227MB/s-227MB/s),
    io=80.0GiB (85.9GB), run=47269-47271msec
- raid1c3 with 2 HDDs and 1 SSD:
  2 x Segate Barracuda ST2000DM008 (2TB)
  1 x Crucial CT256M550SSD1 (256GB)
  * pid policy
    READ: bw=1843MiB/s (1932MB/s), 230MiB/s-230MiB/s (242MB/s-242MB/s),
    io=80.0GiB (85.9GB), run=44449-44450msec
  * roundrobin
    READ: bw=2485MiB/s (2605MB/s), 311MiB/s-311MiB/s (326MB/s-326MB/s),
    io=80.0GiB (85.9GB), run=32969-32970msec

To measure the performance of each policy and find optimal penalty
values, I created scripts which are available here:

https://gitlab.com/vadorovsky/btrfs-perf
https://github.com/mrostecki/btrfs-perf

Michal Rostecki (6):
  btrfs: Add inflight BIO request counter
  btrfs: Store the last device I/O offset
  btrfs: Add stripe_physical function
  btrfs: Check if the filesystem is has mixed type of devices
  btrfs: sysfs: Add directory for read policies
  btrfs: Add roundrobin raid1 read policy

 fs/btrfs/ctree.h   |   3 +
 fs/btrfs/disk-io.c |   3 +
 fs/btrfs/sysfs.c   | 144 ++++++++++++++++++++++++++----
 fs/btrfs/volumes.c | 218 +++++++++++++++++++++++++++++++++++++++++++--
 fs/btrfs/volumes.h |  22 +++++
 5 files changed, 366 insertions(+), 24 deletions(-)

-- 
2.30.0