[dm-devel] [PATCHv5 00/14] dm-zoned: metadata version 2

[Damien Le Moal]

On 2020/05/14 9:22, Martin K. Petersen wrote:
> 
> Damien,
> 
>> Any idea why the io_opt limit is not set to the physical block size
>> when the drive does not report an optimal transfer length ? Would it
>> be bad to set that value instead of leaving it to 0 ?
> 
> The original intent was that io_opt was a weak heuristic for something
> being a RAID device. Regular disk drives didn't report it. These days
> that distinction probably isn't relevant.
> 
> However, before we entertain departing from the historic io_opt
> behavior, I am a bit puzzled by the fact that you have a device that
> reports io_opt as 512 bytes. What kind of device performs best when each
> I/O is limited to a single logical block?
> 

Indeed. It is an NVMe M.2 consumer grade SSD. Nothing fancy. If you look at
nvme/host/core.c nvme_update_disk_info(), you will see that io_opt is set to the
block size... This is probably abusing this limit. So I guess the most elegant
fix may be to have nvme stop doing that ?


-- 
Damien Le Moal
Western Digital Research