Max phy I/O size on Hp-UX

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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