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

[Hannes Reinecke]

On 5/14/20 2:55 AM, Damien Le Moal wrote:
> 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 ?
> 
> 
Yes, I guess that would be the best approach. If the driver doesn't 
report it we shouldn't make up any values but rather leave it at '0'.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
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