RE: Max phy I/O size on Hp-UX

Hi Prasad,

A new tuneable scsi_maxphys was introduced in patch PHKL_13552 in response to SR 4701376087.
The patch notes say ...

          This value was previously fixed at 1MB (1048576).  On
          V-Class systems, this value can be set as high as 32MB
          (33554432) for large raw SCSI disk transfers.  Most
          systems won't need the value set this high, so the
          recommended value should be set to the SCSI tape max
          of 16M-1 (16777215) for V-Class.  Non V-Class systems
          should remain with the system default of 1048576.  To
          enable large record support for SCSI tape drives with
          the stape driver, the new st_large_recs tunables must
          also be set to 1.

As you can see the issue here is tape I/O, not disk I/O, and although the driver limitation mentioned in param.h has been relaxed (for tape I/O) I think that it is effectively unchanged for disk I/O because of the LVM logical track group size restriction.

@   Regards,
@   Steve Adams
@   http://www.ixora.com.au/
@   http://www.christianity.net.au/

-----Original Message-----
[mailto:Prasada.Gunda1_at_hartfordlife.com] Sent: Friday, 23 March 2001 7:17
To: Multiple recipients of list ORACLE-L

This question regarding max physical I/O size for hp-ux v11.

When we were talking to unix sys admin, she searched in HP knowledge base and found that 256k is the maximum
physical I/O for hp-ux. When I was looking at Q&A on Steve Adams's web site, the answer is 1M for max physical i/o size for hp-ux.

Steve, Could you please let me know if I am missing some thing here.

>From Steve Adam's Web site:

 Maximum I/O size
4 January 2000

What is the maximum I/O size (MAXPHYS) nowadays on HP-UX and Solaris?

MAXPHYS has long been fixed at 64K on Solaris, and 256K on HP-UX. However, from
Solaris 2.6 it is defined in /etc/system and defaults to 128K. On HP-UX 11 it
now defaults to 1M. However, I think the LVM layer still constrains I/O operations
to a single logical track group, which is 256K, so the higher MAXPHYS only applies
if you are not using LVM, which is most unusual. Of course, these large physical
I/O sizes are only possible if you are using raw or direct I/O. Perhaps more importantly, there is an internal Oracle kernel constant (SSTIOMAX)
that limits I/O operations to 512K.

>From Knowledge base on HP Web site

What is MAXPHYS for HP-UX systems?

DocId:                              KBRC00003216
Updated:                         7/31/00 7:37:00 AM

PROBLEM
What is the the maximum size of a single I/O (or MAXPHYS) that can be issued to a device on an HP-UX system?

RESOLUTION
MAXPHYS is not a tunable on HP-UX systems but it defaults to 256KB. This can be seen in one of the system header files:

1. vi /usr/include/machine/param.h
2. Search for MAXPHYS

This should return:
#define MAXPHYS (256 * 1024) /* Maximum size of physical I/O transfer */
So MAXPHYS is 256KB. This is not tunable to larger value due to driver restrictions.

Thanks in advance for all your comments.

Regards,
Prasad

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