ZAC & ZBC - reads to unwritten logical blocks / sectors

Ballard, Curtis C (HP Storage) curtis.ballard at hp.com
Thu May 28 09:15:18 PDT 2015


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Joe,
Could you elaborate on items a) and b) in your list?  Maybe there is some
T13/ZAC difference that affects these that isn't obvious from ZBC.
a)	Reads issued with Forced Unit Access: ZBC as you point out specifies
that reads to unwritten sectors return the initialization pattern however
there is no media behind unwritten sectors so it looks to me like the options
are returning the initialization pattern or terminating the command with an
error for attempting to read an unwritten LBA.	I don't see either specified
in ZBC either so would have to assume that the initialization pattern should
be returned and would prefer that be clearly required.
b)	Reads issued to unmapped sectors: ZBC prohibits unmapped sectors for
Host Managed drives. If ZAC also prohibits unmapped sectors for Host Managed
then this looks like it becomes a Host Aware, Sequential Writes Preferred
zone question and looks a lot like the FUA case where reads are issued
without media backing those reads but in this case I believe the logical
block provisioning behavior has to be followed which would be returning a
vendor specific pattern, all zeros, or a fixed non-zero pattern as specified
in SBC4r07b table 10.  I think the 'initialization pattern' for the ZAC/ZBC
device would have to match the 'provisioning initialization pattern'.  That
should probably be specified somewhere.
Curtis Ballard
Hewlett Packard
From: owner-t10 at t10.org [mailto:owner-t10 at t10.org] On Behalf Of Joe Breher
Sent: Thursday, May 28, 2015 8:50 AM
To: T10 org; T13 at T13.org
Subject: ZAC & ZBC - reads to unwritten logical blocks / sectors
In yesterday's ZAC webex, the group collectively identified several holes in
the specification of behavior which include:
a) reads issued with Forced Unit Access;
b) reads issued to unmapped sectors; or
c) what is returned for PI,
for 'unwritten' sectors (i.e. not written since the last reset write pointer
operation to that zone) in:
a) sequential write preferred zones; or
b) sequential write required zones for which the URSWRZ bit (i.e. UR bit) is
set.
The participants on the call issued themselves action items to find out what
their organizations would prefer for these cases.
This email is to report HGST's collective wisdom on the matter. After some
internal discussion, we believe the most logical architectural choices are:
a) return 'initialization pattern' (as specified elsewhere - e.g. zbc-r03
4.3.3.1) for FUA reads;
b) return 'initialization pattern' for unmapped sector reads; and
c) return all F's for PI for sector reads,
for 'unwritten' sectors (i.e. not written since the last reset write pointer
operation to that zone) in:
a) sequential write preferred zones; or
b) sequential write required zones for which the URSWRZ bit (i.e. UR bit) is
set.
Thanks -
Joe Breher
Storage Architecture Technologist
Standards Setting Organization
San Jose Research Center
HGST, a Western Digital company
(478) 2-Breher
(478) 227-3437
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