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

[Anand Jain]

On 10/02/2021 04:30, Michal Rostecki wrote:
> 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

 Both of the above test cases are sequential. How about some random IO
 workload?

 Also, the seek time for non-rotational devices does not exist. So it is
 a good idea to test with ssd + nvme and all nvme or all ssd.

> 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

These patches look good. But as only round-robin policy requires
to monitor the inflight and last-offset. Could you bring them under
if policy=roundrobin? Otherwise, it is just a waste of CPU cycles
if the policy != roundrobin.

>    btrfs: Add stripe_physical function
>    btrfs: Check if the filesystem is has mixed type of devices

Thanks, Anand

>    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(-)