Re: [PATCHv3] scsi: disable automatic target scan

[Hannes Reinecke <hare@suse.de>]

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
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