max_q_depth escsi_maxphys

[xin_jerry]

We use 11.31 system in Superdome. we want to adjust max_q_depth escsi_maxphys parameter to get higher performance. But we don't know depend on what to change it and change to what value.
Could you help us, thanks.

[Turgay Cavdar]

For max_q_depth and escsi_maxphys look at http://docs.hp.com/en/11iv3IOPerf/IOPerformanceWhitePaper.pdf page 9.

Generally storage providers/administrators tell us the optimum value for max_q_depth.

[xin_jerry]

we changed the parameter as followed:
scsimgr save_attr â a escsi_maxphys=1048575
scsimgr save_attr â a max_q_depth=64 â D /dev/rdisk/**

But the performance was bad,when press was heavy, it seemed good. whne the press was less, the process also runned slowly.

Could any body tell me the usage of escsi_maxphys, and why need to modify it?

thanks a lot

[R.K. #]

Hi Jerry,

escsi_maxphys: Maximum I/O size allowed by the SCSI subsystem in 4KB units. Range: 64 - 1048575

Some of the many tunables that can be used for I/O performance are things like escsi_maxphys and the max_q_depth setting. escsi_maxphys controls the max I/O size and has been increased from 1 MB in 11i v2 to 2 MB in 11i v3. The use of a larger max I/O size allows for more efficient I/O performance for coallesced and sequential IOs. The max_q_depth setting is a key scsimgr attribute that allows for an increased number of concurrent I/Os to be issues on a per path basis. The defaiult max_q_depth value is set to eight, and with the use of larger LUN capacities, or even otherwise, a system admin may want to look at the max_q_depth settings and see if their environment can gain in performance from an increase of max_q_depth.


Hope this makes things a bit clear..

[xin_jerry]

thanks for your reply.
escsi_maxphys controls the max I/O size and has been increased from 1 MB in 11i v2 to 2 MB in 11i v3. It's difficult to understand.
escsi_maxphys max value is 1048575. How it controls max I/O. escsi_maxphys default value is 512. If we use max value, what will happen to system?

[Turgay Cavdar]

Hi,
As in the scsimgr man page escsi_maxphys value is :Maximum I/O size allowed by he SCSI subsystem in 4KB units. So 512x4k=2MB.

If you are using VXFS then you already have a limit of max 1MB IO, so it is not meaningfull to tune escsi_maxphys. If you are using disk as a raw devices, or LVM volumes as a raw devices, then you might gain some benefits when doing sequential IO.