Re: [linus:master] [block] e70c301fae: stress-ng.aiol.ops_per_sec 49.6% regression

Christoph Hellwig
2025-01-03T06:49:48Z

On Thu, Jan 02, 2025 at 10:49:41AM +0100, Niklas Cassel wrote:
> > > from below information, it seems an 'ahci' to me. but since I have limited
> > > knowledge about storage driver, maybe I'm wrong. if you want more information,
> > > please let us know. thanks a lot!
> >
> > Yes, this looks like ahci. Thanks a lot!
>
> Did this ever get resolved?
>
> I haven't seen a patch that seems to address this.
>
> AHCI (ata_scsi_queuecmd()) only issues a single command, so if there is any
> reordering when issuing a batch of commands, my guess is that the problem
> also affects SCSI / the problem is in upper layers above AHCI, i.e. SCSI lib
> or block layer.

I started looking into this before the holidays. blktrace shows perfectly
sequential writes without any reordering using ahci, directly on the
block device or using xfs and btrfs when using dd. I also started
looking into what the test does and got as far as checking out the
stress-ng source tree and looking at stress-aiol.c. AFAICS the default
submission does simple reads and writes using increasing offsets.
So if the test result isn't a fluke either the aio code does some
weird reordering or btrfs does.

Oliver, did the test also show any interesting results on non-btrfs
setups?