[SunHELP] large I/O transfers

[DAUBIGNE Sebastien - BOR ( SDaubigne@bordeaux-bersol.sema.slb.com )]

Basically there are I/O size limits in any level of the OS :

[   Volume manager (VxVM/DiskSuite)   ->  ]  Core Kernel -> sd/ssd drivers


I/O size limit in the Core Kernel :

>From 'man fcaw' (available with JNI cards) :

==========
maxphys.
     Determines the maximum number of bytes that can be transferred
     per  SCSI  transaction. This variable is set in the /etc/system file:

          set maxphys = nnn

     The default value is 131072.  If a request size exceeds this
     value,  the  read  or write will be broken up into more than
     one request with each  request  not  exceeding  the  default
     value.  To achieve maximum bandwidth, set maxphys to 8 mega-
     bytes (8388608).
============


I/O size limit in the sd/ssd drivers:

In case you put the value beyond 1 Mb as stated in the "fcaw" manpage, you
must also be aware that there is another undocumented 1Mb limit in the
'sd/ssd' drivers, as stated in the following Sun bug report (4310826) :

============
As for I/Os larger than 1MB the next obstacle is the target drivers sd and
ssd which limit I/O size to 1MB. They have undocumented properties
sd_max_xfer_size and ssd_max_xfer_size which need to be specified in
/etc/sd.conf, /etc/ssd.conf. I/Os larger than 1MB don't buy much until the
disk drives have more than 1MB cache which is the case with HW RAID devices.
============

This means setting "maxphys" beyond 1Mb will not help if you don't modify
the "sd_max_xfer_size" parameter also.


I/O size limit in the Volume Managers (if installed) :

In case you use Veritas VxVM or Sun DiskSuite, be aware that they also have
their internal limit (well, who hasn't any ?) :

For VxVM :   vxio:vol_maxio (value in 512k-blocs, default 512 blocs = 256
Kb)
For DiskSuite :   md:md_paxphys (value bytes, default 131072 bytes= 128 Kb)

---
Sebastien DAUBIGNE
sdaubigne at bordeaux-bersol.sema.slb.com <mailto:sebastien.daubigne at sema.fr>
- (+33)5.57.26.56.36
SchlumbergerSema - Sema Global Services - DW/Pessac

	-----Message d'origine-----
	De:	Markham, Richard [SMTP:RMarkham at hafeleamericas.com]
	Date:	vendredi 19 avril 2002 15:23
	:	Sunhelp (E-mail)
	Objet:	[SunHELP] large I/O transfers

	While reading a document I ran across a section which
	talked about the considering the 'maximum transfer size'
	of the OS.  Other documents have called this max_io_size.
	It states that some OS's cannot transfer more than 64k
	or 128k in a given I/O.  One document mentions that
	Solaris can be configured for up to 1mb I/O sizes. Does
	anyone have information on this?
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