EMC queue depth

[Tim Nelson]

Interesting Doc..
Any idea of what the effects to an application or system if the default queue depth was 8 ?
Is this a system throttle and once >8 application IO is told to wait but not error ?
I see the other extreme in the doc if set too high then the Array would return an error.
Is this analagy correct ?

[Alzhy]

Tim..

Yes your analogy would be correct. And yes .. coupled with your Volume Manager (LVM+VxVM) and other SCSI tunables - Q Depth can be a system I/O throttle of sorts.

"High" Q Depth Settings (in my standards q depths greater than 16) mostly apply to environments that adopt very large LUN size standards and adopt a policy of a Filesystem or Server Volume Unit per LUN.

For XP 1024 (or XP12000)? The per port maximum is I think still 1024. That is why for this kind of array - I always stripe accross LUNs presented on different front-end ports and still bump up q depth from the 11i default of 16. Hitachi suggested staying at 8 on our 9960 (XP512 equivalent)..