[wtf] vtscsi aka virtio_scsi does not see disk at target 7

[Andriy Gapon]

By the way, quite a long time ago, on Wed Sep 26 00:57:10 UTC 2012 Jim Harris
<jimharris at freebsd.org> suggested this:
> I'm wondering if the scsi_xpt.c code
> should instead (or also) ignore the initiator_id if cpi->transport ==
> XPORT_SAS?  initiator_id isn't really valid for SAS transport.

I think that that proposal still makes sense and we should go for it.
Any objections?


On 16/01/2020 17:27, Andriy Gapon wrote:
> 
> I think I see what's going on, but it was a WTF moment for me.
> 
> We use virtio_scsi driver on GCE VMs.  We typically configure VMs with 6 or less
> disks, but recently we started to use VMs with 8+ disks.  On some of them, after
> a reboot, one disk is missing. Here is an example:
> 
> # camcontrol devlist -v
> scbus0 on vtscsi0 bus 0:
> <Google PersistentDisk 1>          at scbus0 target 1 lun 0 (pass0,da0)
> <Google PersistentDisk 1>          at scbus0 target 2 lun 0 (pass1,da1)
> <Google PersistentDisk 1>          at scbus0 target 3 lun 0 (pass2,da2)
> <Google PersistentDisk 1>          at scbus0 target 4 lun 0 (pass3,da3)
> <Google PersistentDisk 1>          at scbus0 target 5 lun 0 (pass4,da4)
> <Google PersistentDisk 1>          at scbus0 target 6 lun 0 (pass5,da5)
> <Google PersistentDisk 1>          at scbus0 target 8 lun 0 (pass6,da6)
> <Google PersistentDisk 1>          at scbus0 target 9 lun 0 (pass7,da7)
> 
> We noticed that it is always target 7 that is missing.
> 
> After looking at the code, the problem is kind of obvious.
> scsi_scan_bus:
>                 for (i = low_target; i <= max_target; i++) {
>                         cam_status status;
> 
>                         if (i == initiator_id)
>                                 continue;
> 
> vtscsi_cam_path_inquiry:
>         cpi->initiator_id = VTSCSI_INITIATOR_ID;
> 
> /*
>  * Specification doesn't say, use traditional SCSI default.
>  */
> #define VTSCSI_INITIATOR_ID     7
> 
> From what I see, on GCE at least, target 7 is a valid target and it is not
> reserved for the initiator.  At the same time, I see that target 0 is never used
> for disks.  Also, it seems that virtio configuration gives 253 as the maximum
> target ID on those instances.
> 
> I would appreciate any suggestions on what to use for initiator_id in this
> driver.  Thanks!
> 


-- 
Andriy Gapon