Re: [PATCH] sd: Limit WRITE SAME / WRITE SAME(16) w/UNMAP length for certain devices

["Ewan D. Milne" <emilne@redhat.com>]

On Mon, 2017-09-25 at 21:46 -0400, Martin K. Petersen wrote:
> Ewan,
> 
> > Some devices do not support a WRITE SAME / WRITE SAME(16) with the
> > UNMAP bit set up to the length specified in the MAXIMUM WRITE SAME
> > LENGTH field in the block limits VPD page (or, the field is zero,
> > indicating there is no limit).  Limit the length by the MAXIMUM UNMAP
> > LBA COUNT value.  Otherwise the command might be rejected.
> 
> From SBC4:
> 
>   "A MAXIMUM UNMAP LBA COUNT field set to a non-zero value indicates the
>   maximum number of LBAs that may be unmapped by an UNMAP command"
> 
> Note that it explicitly states "UNMAP command" and not "unmap
> operation".
> 
>   "A MAXIMUM WRITE SAME LENGTH field set to a non-zero value indicates
>   the maximum number of contiguous logical blocks that the device server
>   allows to be unmapped or written in a single WRITE SAME command."
> 
> It says "unmapped or written" and "WRITE SAME command".
> 
> The spec is crystal clear. The device needs to be fixed. We can
> blacklist older firmware revs.
> 

Yes, I know that is what SBC-4 says, and I agree that the devices
are not conforming.  Unfortunately, I've come across 3 different
arrays now from 3 different manufacturers that exhibit this behavior.

cc: Fred Knight for his opinion on this (NetApp was not one of the
arrays that I've run into, though).

-Ewan