I tried your samba4 version now and it gave me the same poor read performance. I tested the device with fio, which gave me about 80 MB/s read performance, if this is true it is not the device it self that causes the problem.
fio --rw=read --name=test --size=1000M --filename=/dev/sda
test: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=psync, iodepth=1
fio-3.13
Starting 1 process
Jobs: 1 (f=1): [R(1)][100.0%][r=79.4MiB/s][r=20.3k IOPS][eta 00m:00s]
test: (groupid=0, jobs=1): err= 0: pid=3435: Thu Feb 28 16:57:26 2019
read: IOPS=20.6k, BW=80.4MiB/s (84.3MB/s)(1000MiB/12432msec)
clat (usec): min=8, max=13177, avg=24.56, stdev=76.31
lat (usec): min=10, max=13181, avg=28.69, stdev=76.95
clat percentiles (usec):
| 1.00th=[ 10], 5.00th=[ 13], 10.00th=[ 18], 20.00th=[ 18],
| 30.00th=[ 18], 40.00th=[ 19], 50.00th=[ 19], 60.00th=[ 19],
| 70.00th=[ 19], 80.00th=[ 20], 90.00th=[ 21], 95.00th=[ 22],
| 99.00th=[ 273], 99.50th=[ 297], 99.90th=[ 437], 99.95th=[ 519],
| 99.99th=[ 2147]
bw ( KiB/s): min=68798, max=94064, per=99.92%, avg=82301.79, stdev=4312.91, samples=24
iops : min=17199, max=23516, avg=20575.33, stdev=1078.30, samples=24
lat (usec) : 10=2.75%, 20=86.46%, 50=7.79%, 100=0.51%, 250=0.87%
lat (usec) : 500=1.54%, 750=0.04%, 1000=0.01%
lat (msec) : 2=0.01%, 4=0.01%, 10=0.01%, 20=0.01%
cpu : usr=39.54%, sys=57.51%, ctx=1244, majf=0, minf=25
IO depths : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0%
submit : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
complete : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
issued rwts: total=256000,0,0,0 short=0,0,0,0 dropped=0,0,0,0
latency : target=0, window=0, percentile=100.00%, depth=1
Run status group 0 (all jobs):
READ: bw=80.4MiB/s (84.3MB/s), 80.4MiB/s-80.4MiB/s (84.3MB/s-84.3MB/s), io=1000MiB (1049MB), run=12432-12432msec
Disk stats (read/write):
sda: ios=3984/0, merge=0/0, ticks=4830/0, in_queue=4820, util=37.28%
I am going to test it with btrfs and etx2 now to see wether it has anything to to with the filesystem.
Any other suggestions?