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scanning for LUNs

DISCUSSION K. Y. Srinivasan 3 participants 5 messages Apr 4, 2013 – Apr 8, 2013 SCSI

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K. Y. Srinivasan

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#0 lore raw

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

#1 lore raw
Quoted 12 lines · 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)
Quoted 12 lines

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

author
#2 lore raw
Quoted 50 lines

-----Original Message-----
From: James Bottomley [mailto:[email protected]]
Sent: Thursday, April 04, 2013 11:15 AM
To: KY Srinivasan
Cc: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; linux- [email protected]
Subject: Re: scanning for LUNs

Quoted 9 lines · 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) {

Quoted 15 lines
                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:

Quoted 6 lines

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

Quoted 7 lines

James

James

#3 lore raw
Quoted 50 lines · KY Srinivasan
Quoted 50 lines

-----Original Message-----
From: James Bottomley [mailto:[email protected]]
Sent: Thursday, April 04, 2013 11:15 AM
To: KY Srinivasan
Cc: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; linux- [email protected]
Subject: Re: scanning for LUNs

Quoted 9 lines · 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) {

Quoted 15 lines
                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:

Quoted 9 lines
Quoted 6 lines

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

Signature · 8 lines
Dr. Hannes Reinecke		      zSeries & Storage
[email protected]			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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author
#4 lore raw
Quoted 71 lines

-----Original Message-----
From: Hannes Reinecke [mailto:[email protected]]
Sent: Monday, April 08, 2013 10:42 AM
To: KY Srinivasan
Cc: James Bottomley; [email protected]; linux- [email protected]; [email protected]; [email protected]; [email protected]; [email protected]
Subject: Re: scanning for LUNs

Quoted 11 lines · KY Srinivasan
Quoted 11 lines

-----Original Message-----
From: James Bottomley [mailto:[email protected]]
Sent: Thursday, April 04, 2013 11:15 AM
To: KY Srinivasan
Cc: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; linux- [email protected]
Subject: Re: scanning for LUNs

Quoted 2 lines
Quoted 2 lines · K. Y. Srinivasan
Quoted 1 line

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

function

Quoted 25 lines
Quoted 25 lines
Quoted 8 lines

__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) {

Quoted 15 lines
                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.

Quoted 7 lines
Quoted 7 lines

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,

Quoted 12 lines
Quoted 11 lines

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

Quoted 8 lines
Quoted 2 lines

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

Quoted 10 lines

Cheers,

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