How do I use NCQ of Intel X25-E(SSD) on ahci(4)?

[Ivan Voras]

2009/11/18 Alexander Motin <mav at freebsd.org>:
> Ivan Voras wrote:

>> I know next to nothing about AHCI and drivers so this might be obviously
>> wrong but wouldn't a quick (i.e. MFC-able) obvious temporary fix be to say
>>
>> numslots = min(get_minimum_tags_of_all_drives(), ...)
>> ?
>
> Problem is that SIM driver has no idea about devices capabilities, and
> also doesn't have method to resize queue after attach. In SCSI case,
> tags are random and only simultaneous number of request is limited, and
> this is handled fine by CAM. SATA NCQ is more restrictive, allowing to
> use only tags 0..(N-1). I am planning to make XPT inform SIM about
> supported tags for each device, to allow SIM to use that information
> while scheduling requests. I didn't do it yet, just because most of
> devices able to handle all 32 tags possible on SATA. This Intel SSD is
> one of rare exceptions.

Ok, (still thinking about something that could be MFC-able in an
emergency), how about adding a loader tunable integer instead of "32"?