zbc theory of operation

Ralph Weber Ralph.Weber at wdc.com
Wed Aug 6 08:07:08 PDT 2014

* From the T10 Reflector (t10 at t10.org), posted by:
* Ralph Weber <Ralph.Weber at wdc.com>
Dear Dr. Reinecke,
Thank you for your diligence and patience regarding this discussion.
One last point surfaced after the previous message as sent. Although the new
thought has no direct bearing on the discussion, it might be of interest
The zero-based Conventional Zone concept provides an additional service to
the booting operating system. The LBAs in the Conventional Zone have the most
desirable properties for recording the information normally associated with a
swap file.
I suspect the ZBC designers are more than a little proud of the way that this
structure kills two birds with one stone.
All the best,
From: Hannes Reinecke [hare at suse.de]
Sent: Wednesday, August 06, 2014 9:55 AM
To: Ralph Weber; t10 at t10.org
Subject: Re: zbc theory of operation
On 08/06/2014 04:09 PM, Ralph Weber wrote:
> * From the T10 Reflector (t10 at t10.org), posted by:
> * Ralph Weber <Ralph.Weber at wdc.com>
> *
> As I understand the theory, a host that does not want to participate in
 > of the ZBC device should require either Host Aware support or
what the cited article
 > calls "drive-managed SMR", neither one of which forces the host
to know the full map
 > at any time (including boot time). Of course, these models
include the potential for
 > reduced read/write performance when the drive is busy pounding
the round peg in a
 > square hole.
Yes, I am aware of that.
> Disk manufacturers have every reason to expect extra effort from any host
that willingly
 > picks the Host Managed ZBC model as a way to stabilized and/or
improve performance.
 > Surely, the expected effort ought to reasonably include
maintaining a full zone map
 > in at least the file system, even if doing so increases the
consumption of kernel memory.
 > The tradeoff is between drive resources and host resources, and
additional resources means
 > additional money or time costs at either end of the wire. Yes,
the old saw is true: "time=money".
> Nonetheless, a manufacturer of disk drives would be reckless if they
totally ignored
 > boot time issues. This probably explains why all the disk
manufacturers of which I am
 > aware are planning to build at least one product where a
Conventional Zone (see ZBC)
 > occupies a suitably large number of LBAs starting at LBA 0. Some
are even planning to
 > include only these Conventional LBAs in the range of available
LBAs returned by the
 > READ CAPACITY command (see SBC-4), but that is a different kettle
of fish.
> Regardless of the disk implementation details, the additional boot time
 > to determine the size of the zero-based Conventional Zone will be
very small since
 > almost no time is required to transfer the REPORT ZONES
information for a single
 > zone that starts at LBA 0. Furthermore, the use of LBA 0 as the
Starting LBA for
 > a REPORT ZONES command cannot possibly result in a CHECK CONDITION.
Ok. That explains things (a bit).
> Having determined the size of the zero-based Conventional Zone, the host
can continue
 > the booing process without further delays, using the established
size as the operational
 > capacity of the disk. The work necessary to configure the
remainder of the disk
 > can proceed in the background while other booting activities
continue unhindered.
> To be sure, the zero-based Conventional Zone technique is not explicitly
 > in ZBC. Since not all disks are boot disks, the T10 position is
that such a
 > description is not appropriate normative content for ZBC. This
absence is expect
 > to have no effect on disk manufacturers who want to sell drives
to the lucrative
 > boot-device market.
Ok, thanks for the explanation.
Dr. Hannes Reinecke		      zSeries & Storage
hare at suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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