Re: [PATCHv3] scsi: disable automatic target scan

[Laurence Oberman <loberman@redhat.com>]

Hello Hannes

Please share latest scripts and an example of how you are using them.
I have some scripts from last November, that you posted but I am sure they have changed.
If not then I will modify them as appropriate, just let me know.

I have added the patches and booted the system set to async, so before I boot with scsi_mod.scan=manual want to prepare my test system.
This feature may be a very useful feature we would want to include in RHEL as we struggle with large LUN boot configurations all the time.
When you have time and thanks

Laurence Oberman
Principal Software Maintenance Engineer
Red Hat Global Support Services

----- Original Message -----
From: "Hannes Reinecke" <hare@suse.de>
To: "Bart Van Assche" <bart.vanassche@sandisk.com>, "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: "Christoph Hellwig" <hch@lst.de>, "James Bottomley" <james.bottomley@hansenpartnership.com>, linux-scsi@vger.kernel.org
Sent: Monday, March 21, 2016 3:15:10 AM
Subject: Re: [PATCHv3] scsi: disable automatic target scan

On 03/21/2016 02:24 AM, Bart Van Assche wrote:
> On 03/19/16 08:18, Hannes Reinecke wrote:
>> On 03/18/2016 10:56 PM, Bart Van Assche wrote:
>>> On 03/17/2016 12:39 AM, Hannes Reinecke wrote:
>>>> On larger installations it is useful to disable automatic LUN
>>>> scanning, and only add the required LUNs via udev rules.
>>>> This can speed up bootup dramatically.
>>>>
>>>> This patch introduces a new scan module parameter value 'manual',
>>>> which works like 'none', but can be overriden by setting the
>>>> 'rescan'
>>>> value from scsi_scan_target to 'SCSI_SCAN_MANUAL'.
>>>> And it updates all relevant callers to set the 'rescan' value
>>>> to 'SCSI_SCAN_MANUAL' if invoked via the 'scan' option in sysfs.
>>>
>>> Hello Hannes,
>>>
>>> Will setting scsi_scan_type to 'manual' allow a system to boot
>>> from a
>>> SCSI disk? If not, are there alternatives to this approach? Would
>>> it be
>>> a valid alternative to e.g. introduce a new threshold parameter such
>>> that only LUN numbers below this threshold are scanned during boot?
>>>
>> I have a patch for dracut, which will generate udev rules for all
>> devices required for mounting the root fs.
>> Once the system is booted properly I've got another patch for systemd
>> which switches back to 'normal' scanning (ie by writing 'sync' into
>> /sys/modules/scsi_mod/parameters/scan) and rescan all scsi hosts.
>>
>> With that there's no need to have any arbitrary limits; only the
>> necessary devices are enabled during boot.
> 
> Hello Hannes,
> 
> That sounds like a really interesting approach. Will this approach
> also work if the SCSI host and/or LUN numbers change during a reboot?
> 
It's independent on the SCSI host as it just looks for the rport ID
(FC WWPN, SAS ID, or iSCSI target name). The LUN number, however, is
fixed; the whole point of this exercise is that you want to blank
out individual LUNs behind a given target.
Hence you need to able to address the LUNs in the first place.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare@suse.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton
HRB 21284 (AG Nürnberg)
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