RFC: SCSI UNMAP (TRIM) support

Hi.

I've implemented patch for logical block provisioning (aka UNMAP, TRIM, 
BIO_DELETE) support for the CAM da driver in HEAD and would like to ask 
for review, testing and hardware support information.

Depending on device capabilities I use several different methods to 
implement it. Method can be read/set via kern.cam.da.X.delete_method 
sysctls. Possible values are:
  NONE - no provisioning support reported by the device;
  DISABLE - provisioning support was disabled because of errors;
  ZERO - use WRITE SAME (10) command to write zeroes;
  WS10 - use WRITE SAME (10) command with UNMAP bit set;
  WS16 - use WRITE SAME (16) command with UNMAP bit set;
  UNMAP - use UNMAP command.
Last two methods (UNMAP and WS16) are defined by SBC specification and 
UNMAP method is the most advanced one. The rest of methods I've found 
supported in Linux, and as soon as they were trivial to implement, then 
why not? Hope they will be useful in some cases.

Unluckily at this moment I have no devices reporting parameters of the 
logical block provisioning support via respective VPD pages (0xB0 and 
0xB2). So all info I have/use now is the flag telling whether logical 
block provisioning is supported or not. As result specific methods 
chosen now by trying different ones in order (UNMAP, WS16, DISABLE) and 
checking completion status to fallback if needed. I don't expect 
problems from this, as if something go wrong, it should just disable 
itself. It may disable even too aggressively.

Unlike Linux, which executes each delete with separate request, I've 
implemented here the same request aggregation as implemented in ada 
driver. Tests on SSDs I have show much better results doing it this way: 
above 8GB/s of the linear delete on Intel SATA SSD on LSI SAS HBA (mps).

The patch can be found here:
http://people.freebsd.org/~mav/scsi_unmap.patch

Work sponsored by iXsystems, Inc.

If somebody has SAS SSDs with UNMAP support, I would be grateful for 
additional information about them, such as:
camcontrol cmd da0 -c "12 01 00 00 ff 00" -i 255 - >pages
camcontrol cmd da0 -c "12 01 b0 00 ff 00" -i 255 - >page_b0
camcontrol cmd da0 -c "12 01 b2 00 ff 00" -i 255 - >page_b2
camcontrol cmd da0 -c "9e 10 00 00 00 00 00 00 00 00 00 00 00 ff 00 00" 
-i 255 - >rc16

Thank you!

-- 
Alexander Motin