Re: About scsi device queue depth

John Garry
2021-01-12T09:17:24Z

On 12/01/2021 07:23, Hannes Reinecke wrote:
>>
>> So it seems to me that the block layer is merging more bios per
>> request, as averge sg count per request goes up from 1 - > upto 6 or
>> more. As I see, when queue depth lowers the only thing that is really
>> changing is that we fail more often in getting the budget in
>> scsi_mq_get_budget()->scsi_dev_queue_ready().
>>
>> So initial sdev queue depth comes from cmd_per_lun by default or
>> manually setting in the driver via scsi_change_queue_depth(). It seems
>> to me that some drivers are not setting this optimally, as above.
>>
>> Thoughts on guidance for setting sdev queue depth? Could blk-mq
>> changed this behavior?
>>
> First of all: are these 'real' SAS SSDs?
> The peak at 32 seems very ATA-ish, and I wouldn't put it past the LSI
> folks to optimize for that case :-)
> Can you get a more detailed picture by changing the queue depth more
> finegrained?
> (Will get you nicer graphs to boot :-)

They're HUSMM1640ASS204 - not the fastest you can get today, but still
decent.

I'll see about fine-grained IOPs vs depth results ...

Cheers,
John