Re: per-entity statistics

["Jose R. Santos" <jrs at us dot ibm dot com>]

Martin Peschke wrote:

> Jose R. Santos wrote:
> > Unless you only have a single SCSI card with just one channel, just 
> > getting the LUN is not enough. You need the SCSI host number, channel, 
> > lun, and ID. Here is a bit of code from the trace tool that shows how to 
> > get that information using SystemTap.
> >
> > function log_scsi_iodone_extra(var:long)
> > %{
> > struct scsi_cmnd *cmd = (struct scsi_cmnd *)((long)THIS->var);
> > long long scsi_info;
> >
> > scsi_info = ((cmd->device->host->host_no & 0xFF) << 24) |
> > ((cmd->device->channel & 0xFF) << 16) |
> > ((cmd->device->lun & 0xFF) << 8) |
> > (cmd->device->id & 0xFF);
>
> Well, it is obvious why you have masked, shifted and or'ed
> thoses numbers. But one might get easily in trouble doing this.
> Think about storage area networks and lots of devices being
> attached to a single Linux. For now this code works in
> most environments because the Linux SCSI stack does its
> own compact LUN enumeration. But it's not airtight.
>
> Martin
>
>  
I'll admit that I chose these based on my limited experience with 
storage devices. While connecting a large amount of devices in a single 
Linux system is no rare this is usually (in my experience) not done 
through a single SCSI channel. It would seem odd to me for someone to 
configure more than 256 SCSI device on a single SCSI channel, but it 
could happen. Have you seen environments were this is not the case?

I don't have a problem changing this (especially now that we do not have 
post-processing scripts that depend on the structure of the data).

-JRS