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Re: [dm-devel] SCSI's heuristics for enabling WRITE SAME still need work [was: dm mpath: disable WRITE SAME if it fails]

On 13-09-25 04:52 PM, Bernd Schubert wrote:
On 09/24/2013 03:49 PM, Martin K. Petersen wrote:
"Mike" == Mike Snitzer <snitzer redhat com> writes:

Mike> So are there drives like this?:
Mike> 1) don't support RSOC
Mike> 2) do support WRITE SAME
Mike> 3) do populate VPD page with either WRITE SAME w/ discard bit set
Mike>    or UNMAP?


But again, the fundamental issue here is not the drives. It's the
controller firmware. I am not aware of a single SPI/SAS/FC drive that
does not support at least WRITE SAME(10).

For DIX and T10 PI I have a capabilities mask that each HBA driver fills
out that tells the sd driver what the controller can do. And this is
combined with whatever the drive reports to figure out whether integrity
protection can be enabled.

I have been contemplating doing something similar for "fancy" SCSI
commands. We could have a flag in the scsi host template that controls
whether the device supports WRITE SAME, EXTENDED COPY, etc. The
advantage being that we do the matching at discovery time instead of
once a WRITE SAME is issued.

This would also permit HBA drivers to toggle the feature on a per
instance basis. I.e. if "RAID controller firmware rev is lower than XYZ,
do not support WRITE SAME".

I'll do a PoC later today...


I'm afraid we have another problem. I'm currently working on to get
discard working for our LSI2008 HBAs with attached sata-SSDs and the
heuristics in sd_read_write_same with based on VPD page 0x89 is not
correct for this HBA - its SATL supports write-same (although it does
"Logical block address out of range" at the end of the device, I'm going
to look into this tomorrow).

So allow LSI SATL or remove this check at all?

LSI implement their SATL in firmware inside their HBAs. Given
their latest firmware release name (*Package_P17_IR_IT_Firmware*
dated 9 August 2013) for their SAS-2 family *** you have 17
versions of that firmware potentially out there in the field.
Updating that firmware is a fiddly process; I use a USB stick
with DOS on it! LSI do fix things in their SATL when flaws are
pointed out.

Generally speaking LSI's SATL is pretty good, at least compared
to another SATL I can think of. They are both moving targets,
and move independently.

Doug Gilbert

*** LSI's SAS-3 HBAs are still at "P1" which I assume is the
    first publically released version.

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