zbc theory of operation

Hannes Reinecke hare at suse.de
Wed Aug 6 07:55:42 PDT 2014


* From the T10 Reflector (t10 at t10.org), posted by:
* Hannes Reinecke <hare at suse.de>
*
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
operation
 > 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
required
 > 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
described
 > 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.
Cheers,
Hannes
-- 
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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