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

[Dave Chinner <david@fromorbit.com>]

[cc linux-block]

[original bug report: https://lore.kernel.org/linux-xfs/CAAiJnjoo0--yp47UKZhbu8sNSZN6DZ-QzmZBMmtr1oC=fOOgAQ@mail.gmail.com/ ]

On Sun, May 04, 2025 at 10:22:58AM +0300, Anton Gavriliuk wrote:
> > What's the comparitive performance of an identical read profile
> > directly on the raw MD raid0 device?
> 
> Rocky 9.5 (5.14.0-503.40.1.el9_5.x86_64)
> 
> [root@localhost ~]# df -mh /mnt
> Filesystem      Size  Used Avail Use% Mounted on
> /dev/md127       35T  1.3T   34T   4% /mnt
> 
> [root@localhost ~]# fio --name=test --rw=read --bs=256k
> --filename=/dev/md127 --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=81.4GiB/s][r=334k IOPS][eta 00m:00s]
> test: (groupid=0, jobs=1): err= 0: pid=43189: Sun May  4 08:22:12 2025
>   read: IOPS=363k, BW=88.5GiB/s (95.1GB/s)(2656GiB/30001msec)
>     slat (nsec): min=971, max=312380, avg=1817.92, stdev=1367.75
>     clat (usec): min=78, max=1351, avg=174.46, stdev=28.86
>      lat (usec): min=80, max=1352, avg=176.27, stdev=28.81
> 
> Fedora 42 (6.14.5-300.fc42.x86_64)
> 
> [root@localhost anton]# df -mh /mnt
> Filesystem      Size  Used Avail Use% Mounted on
> /dev/md127       35T  1.3T   34T   4% /mnt
> 
> [root@localhost ~]# fio --name=test --rw=read --bs=256k
> --filename=/dev/md127 --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=41.0GiB/s][r=168k IOPS][eta 00m:00s]
> test: (groupid=0, jobs=1): err= 0: pid=5685: Sun May  4 10:14:00 2025
>   read: IOPS=168k, BW=41.0GiB/s (44.1GB/s)(1231GiB/30001msec)
>     slat (usec): min=3, max=273, avg= 5.63, stdev= 1.48
>     clat (usec): min=67, max=2800, avg=374.99, stdev=29.90
>      lat (usec): min=72, max=2914, avg=380.62, stdev=30.22

So the MD block device shows the same read performance as the
filesystem on top of it. That means this is a regression at the MD
device layer or in the block/driver layers below it. i.e. it is not
an XFS of filesystem issue at all.

-Dave.
-- 
Dave Chinner
david@fromorbit.com