Re: [PATCH] blk-settings: round down io_opt to at least 4K

["Martin K. Petersen" <martin.petersen@oracle.com>]

Mikulas,

> Do you think that SSDs benefit from more alignment than 4K?

It really depends on the SSD.

> If you have some SSD that has higher IOPS with alignment larger than
> 4K, let me know.

Absolutely.

There are devices out there reporting characteristics that are not
intuitively correct. I.e. not obvious powers of two due to internal
striping or allocation units. Some devices prefer an alignment of 768KB,
for instance. Also, wide RAID stripes can end up causing peculiar
alignment to be reported.

Instead of assuming that 4KB is the correct value for all devices, I'd
rather address the cases where one particular device is reporting
something wrong.

I'm willing to entertain having a quirk to ignore reported values of
0xffff (have also seen 0xfffe) for USB-ATA bridge devices. But I don't
think we should blindly distrust what devices using other transport
classes happen to report, it's usually only USB bridges that are spewing
nonsense. Which is why we typically avoid trying to make sense of VPD
pages on USB in the first place.

-- 
Martin K. Petersen	Oracle Linux Engineering