Sequential read from NVMe/XFS twice slower on Fedora 42 than on Rocky 9.5

[Dave Chinner]

On Sun, May 04, 2025 at 12:04:16AM +0300, Anton Gavriliuk wrote:
> There are 12 Kioxia CM-7 NVMe SSDs configured in mdadm/raid0 and
> mounted to /mnt.
> 
> Exactly the same fio command running under Fedora 42
> (6.14.5-300.fc42.x86_64) and then under Rocky 9.5
> (5.14.0-503.40.1.el9_5.x86_64) shows twice the performance difference.
> 
> /mnt/testfile size 1TB
> server's total dram 192GB
> 
> Fedora 42
> 
> [root at localhost ~]# fio --name=test --rw=read --bs=256k
> --filename=/mnt/testfile --direct=1 --numjobs=1 --iodepth=64 --exitall
> --group_reporting --ioengine=libaio --runtime=30 --time_based
> test: (g=0): rw=read, bs=(R) 256KiB-256KiB, (W) 256KiB-256KiB, (T)
> 256KiB-256KiB, ioengine=libaio, iodepth=64
> fio-3.39-44-g19d9
> Starting 1 process
> Jobs: 1 (f=1): [R(1)][100.0%][r=49.6GiB/s][r=203k IOPS][eta 00m:00s]
> test: (groupid=0, jobs=1): err= 0: pid=2465: Sat May  3 17:51:24 2025
>   read: IOPS=203k, BW=49.6GiB/s (53.2GB/s)(1487GiB/30001msec)
>     slat (usec): min=3, max=1053, avg= 4.60, stdev= 1.76
>     clat (usec): min=104, max=4776, avg=310.53, stdev=29.49
>      lat (usec): min=110, max=4850, avg=315.13, stdev=29.82

> Rocky 9.5
> 
> [root at localhost ~]# fio --name=test --rw=read --bs=256k
> --filename=/mnt/testfile --direct=1 --numjobs=1 --iodepth=64 --exitall
> --group_reporting --ioengine=libaio --runtime=30 --time_based
> test: (g=0): rw=read, bs=(R) 256KiB-256KiB, (W) 256KiB-256KiB, (T)
> 256KiB-256KiB, ioengine=libaio, iodepth=64
> fio-3.39-44-g19d9
> Starting 1 process
> Jobs: 1 (f=1): [R(1)][100.0%][r=96.0GiB/s][r=393k IOPS][eta 00m:00s]
> test: (groupid=0, jobs=1): err= 0: pid=15467: Sun May  4 00:00:39 2025
>   read: IOPS=390k, BW=95.3GiB/s (102GB/s)(2860GiB/30001msec)
>     slat (nsec): min=1111, max=183816, avg=2117.94, stdev=1412.34
>     clat (usec): min=81, max=1086, avg=161.60, stdev=19.67
>      lat (usec): min=82, max=1240, avg=163.72, stdev=19.73
> 

Completely latency has doubled on the fc42 kernel. For a read, there
isn't much in terms of filesystem work to be done on direct IO
completion, so I'm not sure this is a filesystem issue...

What's the comparitive performance of an identical read profile
directly on the raw MD raid0 device?

-Dave.
-- 
Dave Chinner
david at fromorbit.com