SUMMARY: max_io_size

Thanks to:

Sebastien Daubigne
Michael Korte


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Question was:
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Hello Admins,
Can any body tell me how to find out the MAX_IO_SIZE in solaris?


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Answer:
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The "maxphys" kernel parameter limits the max. I/O size in the SCSI layer.
Default is 128k.
The "vol_maxio" kernel parameter limits the max. I/O size in the VxVM layer
(if installed). Default is 256k.
The "md_maxphys" kernel parameter limits the max. I/O size in the
DiskSuite/SVM layer (if installed). Default is equal to maxphys.
The "sd_max_xfer_size" module parameter limits the max. I/O size in the SD
module (if installed). Default is 1 Mb.
The " sd_max_xfer_size" module parameter limits the max. I/O size in the SSD
module (if installed). Default is 1 Mb.

There is a good doc on I/O size limits in the following Technote :

-- 08-02-2002

Set the default max. I/O size in Veritas to match/exceed the max. I/O size
that will be used.

in /etc/system, place

set maxphys=1048576
set vxio:vol_maxio=2048
* to allow 1MB I/Os.

vol_maxio is in Veritas units i.e. disk sectors of 512 bytes.
maxphys is in bytes.

We used to have exactly the same issue with DiskSuite, BTW, and needed
to set md:md_maxphys in /etc/system.  However, I understand DiskSuite was
modified to check the kernel maxphys value and use that to override
md_maxphys
if it was larger.  It is convenient to only have to turn one knob.

Maybe vxvm should adopt that strategy ?

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.
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