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disk queue length

李喆
Occasional Visitor

disk queue length

I recently read a perfermance book <>.About 9.8.5 Typical Metric Values:"Disk utilization metrics are misleading because the graph shows only the busiest disk, and displaying it does not mean that it is incapable of more I/O. The physical disk queue is the most meaningful metric: consistent queue lengths as reported by sar or glance that are greater than four are indicators of bottlenecks. Note that queue lengths reported by these tools are the operating system disk queue lengths, not the length of the queue on each individual disk or array. For instance, if the command queue depth of a disk is set to eight and sar reports a disk queue length of four, then there are actually twelve requests waiting to be serviced by the disk."
i look up some material,operating system disk queue lengths is set to used kernel parameter scsi_max_qdepth;the length of the queue on each individual disk is set to use command scsictl.i don't sure that.Please.

2 REPLIES
李喆
Occasional Visitor

Re: disk queue length

I make a test on HP-UX 11.23 PA.
1)SD32#[/]scsictl -c get_lun_parms /dev/rdsk/c0t0d0

LUN LIMITS
----------
flags: 0x0
max queue depth: 8

NEGOTIATED LUN VALUES
---------------------
flags: 0x0
max queue depth: 8
2)set scsi_max _qdepth from 8 to 16
SD32#[/]kctune -B scsi_max_qdepth=16
3)SD32#[/]scsictl -c get_lun_parms /dev/rdsk/c0t0d0

LUN LIMITS
----------
flags: 0x0
max queue depth: 16

NEGOTIATED LUN VALUES
---------------------
flags: 0x0
max queue depth: 16

Turgay Cavdar
Honored Contributor

Re: disk queue length

So i am not sure what is your question but for hp-ux 11.23 scsi_max_qdepth kernel parameters set default global maximum number of I/Os for the device. To set q_depth individually for disk device you can use scsictl command.For example to set q_Depth to 32 for c0t0d0 type:
scsictl -m queue_depth=32 /dev/rdsk/c0t0d0