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SubjectRe: [PATCH 09/14] target/configfs: Expose protection device attributes
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On Mon, 2014-01-13 at 11:27 -0800, Nicholas A. Bellinger wrote:
> On Mon, 2014-01-13 at 10:52 -0800, James Bottomley wrote:
> > On Mon, 2014-01-13 at 10:30 -0800, Nicholas A. Bellinger wrote:
> > > Hey MKP,
> > >
> > > On Fri, 2014-01-10 at 16:01 -0500, Martin K. Petersen wrote:
> > > > >>>>> "nab" == Nicholas A Bellinger <nab@daterainc.com> writes:
> > > >
> > > > nab> This patch adds support for exposing DIF protection device
> > > > nab> attributes via configfs. This includes:
> > > >
> > > > nab> pi_prot_type: Protection Type (0, 1, 3 currently support)
> > > > nab> pi_prot_version: Protection Version (DIF v1 currently supported)
> > > >
> > > > What's DIF v2?
> > > >
> > >
> > > This would be the proposed 16-byte protection scheme for SBC4.
> >
> > What proposed 16 byte scheme? The only DIF proposals I know for SBC-4
> > are 13-185R0 and 12-369R0 and that's a couple of new algorithms and
> > types because we cannot change the 8 byte PI.
> >
>
> Then I'm probably getting the SBC version wrong.. It's the one that
> includes using CRC32C for the block guard, and larger space for
> reference tag as mentioned by MKP.

Well, this isn't reducing my confusion: I think you're referring to
12-369r0 which proposes to *eliminate* the reference tag (by moving it
into the CRC calculation) and use the recovered 4 bytes to expand the
CRC to CRC32 and add two bytes to the application tag, so they both
become 4 bytes long, but the new PI still occupies only 8 bytes on disk.

Perhaps it's also better to clarify the terminology: The PI is composed
of a Guard field, which is the checksum, an application tag, which is
usable by the application for anything it wants and a reference tag
which is designed to be derived from the on-disk location so it can be
used to detect misplaced writes. In Type 1 the reference tag has to be
the lower 31 bits of the LBA and in type 3 it's application defined. In
all current types, the guard is two bytes and the application tag two
bytes.

James




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