scanning for LUNs

scanning for LUNs

[K. Y. Srinivasan]

Here is the code snippet for scanning LUNS (drivers/scsi/scsi_scan.c in function 
__scsi_scan_target()):

        /*
         * Scan LUN 0, if there is some response, scan further. Ideally, we
         * would not configure LUN 0 until all LUNs are scanned.
         */
        res = scsi_probe_and_add_lun(starget, 0, &bflags, NULL, rescan, NULL);
        if (res == SCSI_SCAN_LUN_PRESENT || res == SCSI_SCAN_TARGET_PRESENT) {
                if (scsi_report_lun_scan(starget, bflags, rescan) != 0)
 

So, if we don't get a response while scanning LUN0, we will not use scsi_report_lun_scan().
On Hyper-V, the scsi emulation on the host does not treat LUN0 as anything special and we 
could have situations where the only device under a scsi controller is at a location other than 0
or 1. In this case the standard LUN scanning code in Linux fails to detect this device. Is this
behaviour expected? Why is LUN0 treated differently here. Looking at the scsi spec, I am not sure
if this is what is specified. Any help/guidance will be greatly appreciated.
 
Regards,

K. Y

Re: scanning for LUNs

[James Bottomley]

On Thu, 2013-04-04 at 08:12 -0700, K. Y. Srinivasan wrote:
> Here is the code snippet for scanning LUNS (drivers/scsi/scsi_scan.c in function 
> __scsi_scan_target()):
> 
>         /*
>          * Scan LUN 0, if there is some response, scan further. Ideally, we
>          * would not configure LUN 0 until all LUNs are scanned.
>          */
>         res = scsi_probe_and_add_lun(starget, 0, &bflags, NULL, rescan, NULL);
>         if (res == SCSI_SCAN_LUN_PRESENT || res == SCSI_SCAN_TARGET_PRESENT) {
>                 if (scsi_report_lun_scan(starget, bflags, rescan) != 0)
>  
> 
> So, if we don't get a response while scanning LUN0, we will not use
> scsi_report_lun_scan().
> On Hyper-V, the scsi emulation on the host does not treat LUN0 as
> anything special and we 
> could have situations where the only device under a scsi controller is
> at a location other than 0
> or 1. In this case the standard LUN scanning code in Linux fails to
> detect this device. Is this
> behaviour expected? Why is LUN0 treated differently here. Looking at
> the scsi spec, I am not sure
> if this is what is specified. Any help/guidance will be greatly
> appreciated.

Why don't you describe the problem.  We can't scan randomly a bunch of
LUNs hoping for a response (the space is 10^19).  SAM thinks you use
LUNW for this, but that's not well supported.  We can't annoy USB
devices by probing with REPORT LUNS, so conventionally most arrays
return something for LUN0 even if they don't actually have one (That's
what the peripheral qualifier codes are supposed to be about).  We
translate PQ1 and PQ2 to SCSI_SCAN_TARGET_PRESENT, which means no LUN,
but there is a target to scan here.

If you're sending back an error to an INQUIRY to LUN0, then you're out
of spec.  The SCSI standards say:

        SPC3 6.4.1: In response to an INQUIRY command received by an
        incorrect logical unit, the SCSI target device shall return the
        INQUIRY data with the peripheral qualifier set to the value
        defined in 6.4.2. The INQUIRY command shall return CHECK
        CONDITION status only when the device server is unable to return
        the requested INQUIRY data

James


James

RE: scanning for LUNs

[KY Srinivasan]

> -----Original Message-----
> From: James Bottomley [mailto:jbottomley@parallels.com]
> Sent: Thursday, April 04, 2013 11:15 AM
> To: KY Srinivasan
> Cc: gregkh@linuxfoundation.org; linux-kernel@vger.kernel.org;
> devel@linuxdriverproject.org; ohering@suse.com; hch@infradead.org; linux-
> scsi@vger.kernel.org
> Subject: Re: scanning for LUNs
> 
> On Thu, 2013-04-04 at 08:12 -0700, K. Y. Srinivasan wrote:
> > Here is the code snippet for scanning LUNS (drivers/scsi/scsi_scan.c in function
> > __scsi_scan_target()):
> >
> >         /*
> >          * Scan LUN 0, if there is some response, scan further. Ideally, we
> >          * would not configure LUN 0 until all LUNs are scanned.
> >          */
> >         res = scsi_probe_and_add_lun(starget, 0, &bflags, NULL, rescan, NULL);
> >         if (res == SCSI_SCAN_LUN_PRESENT || res ==
> SCSI_SCAN_TARGET_PRESENT) {
> >                 if (scsi_report_lun_scan(starget, bflags, rescan) != 0)
> >
> >
> > So, if we don't get a response while scanning LUN0, we will not use
> > scsi_report_lun_scan().
> > On Hyper-V, the scsi emulation on the host does not treat LUN0 as
> > anything special and we
> > could have situations where the only device under a scsi controller is
> > at a location other than 0
> > or 1. In this case the standard LUN scanning code in Linux fails to
> > detect this device. Is this
> > behaviour expected? Why is LUN0 treated differently here. Looking at
> > the scsi spec, I am not sure
> > if this is what is specified. Any help/guidance will be greatly
> > appreciated.
> 
> Why don't you describe the problem.  We can't scan randomly a bunch of
> LUNs hoping for a response (the space is 10^19).  SAM thinks you use
> LUNW for this, but that's not well supported.  We can't annoy USB
> devices by probing with REPORT LUNS, so conventionally most arrays
> return something for LUN0 even if they don't actually have one (That's
> what the peripheral qualifier codes are supposed to be about).  We
> translate PQ1 and PQ2 to SCSI_SCAN_TARGET_PRESENT, which means no LUN,
> but there is a target to scan here.
> 
> If you're sending back an error to an INQUIRY to LUN0, then you're out
> of spec.  The SCSI standards say:
> 
>         SPC3 6.4.1: In response to an INQUIRY command received by an
>         incorrect logical unit, the SCSI target device shall return the
>         INQUIRY data with the peripheral qualifier set to the value
>         defined in 6.4.2. The INQUIRY command shall return CHECK
>         CONDITION status only when the device server is unable to return
>         the requested INQUIRY data

Thanks James. I will further investigate the issue on our platform.

Regards,

K. Y
> 
> James
> 
> 
> James
> 
> 

Re: scanning for LUNs

[Hannes Reinecke]

On 04/04/2013 07:12 PM, KY Srinivasan wrote:
> 
> 
>> -----Original Message-----
>> From: James Bottomley [mailto:jbottomley@parallels.com]
>> Sent: Thursday, April 04, 2013 11:15 AM
>> To: KY Srinivasan
>> Cc: gregkh@linuxfoundation.org; linux-kernel@vger.kernel.org;
>> devel@linuxdriverproject.org; ohering@suse.com; hch@infradead.org; linux-
>> scsi@vger.kernel.org
>> Subject: Re: scanning for LUNs
>>
>> On Thu, 2013-04-04 at 08:12 -0700, K. Y. Srinivasan wrote:
>>> Here is the code snippet for scanning LUNS (drivers/scsi/scsi_scan.c in function
>>> __scsi_scan_target()):
>>>
>>>         /*
>>>          * Scan LUN 0, if there is some response, scan further. Ideally, we
>>>          * would not configure LUN 0 until all LUNs are scanned.
>>>          */
>>>         res = scsi_probe_and_add_lun(starget, 0, &bflags, NULL, rescan, NULL);
>>>         if (res == SCSI_SCAN_LUN_PRESENT || res ==
>> SCSI_SCAN_TARGET_PRESENT) {
>>>                 if (scsi_report_lun_scan(starget, bflags, rescan) != 0)
>>>
>>>
>>> So, if we don't get a response while scanning LUN0, we will not use
>>> scsi_report_lun_scan().
>>> On Hyper-V, the scsi emulation on the host does not treat LUN0 as
>>> anything special and we
>>> could have situations where the only device under a scsi controller is
>>> at a location other than 0
>>> or 1. In this case the standard LUN scanning code in Linux fails to
>>> detect this device. Is this
>>> behaviour expected? Why is LUN0 treated differently here. Looking at
>>> the scsi spec, I am not sure
>>> if this is what is specified. Any help/guidance will be greatly
>>> appreciated.
>>
>> Why don't you describe the problem.  We can't scan randomly a bunch of
>> LUNs hoping for a response (the space is 10^19).  SAM thinks you use
>> LUNW for this, but that's not well supported.  We can't annoy USB
>> devices by probing with REPORT LUNS, so conventionally most arrays
>> return something for LUN0 even if they don't actually have one (That's
>> what the peripheral qualifier codes are supposed to be about).  We
>> translate PQ1 and PQ2 to SCSI_SCAN_TARGET_PRESENT, which means no LUN,
>> but there is a target to scan here.
>>
>> If you're sending back an error to an INQUIRY to LUN0, then you're out
>> of spec.  The SCSI standards say:
>>
>>         SPC3 6.4.1: In response to an INQUIRY command received by an
>>         incorrect logical unit, the SCSI target device shall return the
>>         INQUIRY data with the peripheral qualifier set to the value
>>         defined in 6.4.2. The INQUIRY command shall return CHECK
>>         CONDITION status only when the device server is unable to return
>>         the requested INQUIRY data
> 
> Thanks James. I will further investigate the issue on our platform.
> 
Or check if you can use W_LUN for scanning.
I've done a patchset for this (check the mailing list).

Using W_LUN is precisely for this type of setup.

(And would provide me with another scenario for using W_LUNs :-)

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)

RE: scanning for LUNs

[KY Srinivasan]

> -----Original Message-----
> From: Hannes Reinecke [mailto:hare@suse.de]
> Sent: Monday, April 08, 2013 10:42 AM
> To: KY Srinivasan
> Cc: James Bottomley; gregkh@linuxfoundation.org; linux-
> kernel@vger.kernel.org; devel@linuxdriverproject.org; ohering@suse.com;
> hch@infradead.org; linux-scsi@vger.kernel.org
> Subject: Re: scanning for LUNs
> 
> On 04/04/2013 07:12 PM, KY Srinivasan wrote:
> >
> >
> >> -----Original Message-----
> >> From: James Bottomley [mailto:jbottomley@parallels.com]
> >> Sent: Thursday, April 04, 2013 11:15 AM
> >> To: KY Srinivasan
> >> Cc: gregkh@linuxfoundation.org; linux-kernel@vger.kernel.org;
> >> devel@linuxdriverproject.org; ohering@suse.com; hch@infradead.org; linux-
> >> scsi@vger.kernel.org
> >> Subject: Re: scanning for LUNs
> >>
> >> On Thu, 2013-04-04 at 08:12 -0700, K. Y. Srinivasan wrote:
> >>> Here is the code snippet for scanning LUNS (drivers/scsi/scsi_scan.c in
> function
> >>> __scsi_scan_target()):
> >>>
> >>>         /*
> >>>          * Scan LUN 0, if there is some response, scan further. Ideally, we
> >>>          * would not configure LUN 0 until all LUNs are scanned.
> >>>          */
> >>>         res = scsi_probe_and_add_lun(starget, 0, &bflags, NULL, rescan, NULL);
> >>>         if (res == SCSI_SCAN_LUN_PRESENT || res ==
> >> SCSI_SCAN_TARGET_PRESENT) {
> >>>                 if (scsi_report_lun_scan(starget, bflags, rescan) != 0)
> >>>
> >>>
> >>> So, if we don't get a response while scanning LUN0, we will not use
> >>> scsi_report_lun_scan().
> >>> On Hyper-V, the scsi emulation on the host does not treat LUN0 as
> >>> anything special and we
> >>> could have situations where the only device under a scsi controller is
> >>> at a location other than 0
> >>> or 1. In this case the standard LUN scanning code in Linux fails to
> >>> detect this device. Is this
> >>> behaviour expected? Why is LUN0 treated differently here. Looking at
> >>> the scsi spec, I am not sure
> >>> if this is what is specified. Any help/guidance will be greatly
> >>> appreciated.
> >>
> >> Why don't you describe the problem.  We can't scan randomly a bunch of
> >> LUNs hoping for a response (the space is 10^19).  SAM thinks you use
> >> LUNW for this, but that's not well supported.  We can't annoy USB
> >> devices by probing with REPORT LUNS, so conventionally most arrays
> >> return something for LUN0 even if they don't actually have one (That's
> >> what the peripheral qualifier codes are supposed to be about).  We
> >> translate PQ1 and PQ2 to SCSI_SCAN_TARGET_PRESENT, which means no
> LUN,
> >> but there is a target to scan here.
> >>
> >> If you're sending back an error to an INQUIRY to LUN0, then you're out
> >> of spec.  The SCSI standards say:
> >>
> >>         SPC3 6.4.1: In response to an INQUIRY command received by an
> >>         incorrect logical unit, the SCSI target device shall return the
> >>         INQUIRY data with the peripheral qualifier set to the value
> >>         defined in 6.4.2. The INQUIRY command shall return CHECK
> >>         CONDITION status only when the device server is unable to return
> >>         the requested INQUIRY data
> >
> > Thanks James. I will further investigate the issue on our platform.
> >
> Or check if you can use W_LUN for scanning.
> I've done a patchset for this (check the mailing list).
> 
> Using W_LUN is precisely for this type of setup.
> 
> (And would provide me with another scenario for using W_LUNs :-)

Thanks Hannes. What is the status on this patch; is it planned for the next upstream release?

Regards,

K. Y
> 
> Cheers,
> 
> Hannes
> --
> Dr. Hannes Reinecke		      zSeries & Storage
> hare@suse.de			      +49 911 74053 688
> SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
> GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
>