linux-kernel - Re: deterministic scsi order with async scan

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Message-ID: <alpine.DEB.1.10.0907161245470.9159@asgard>
Date:	Thu, 16 Jul 2009 12:48:06 -0700 (PDT)
From:	david@...g.hm
To:	James Bottomley <James.Bottomley@...senPartnership.com>
cc:	James Smart <James.Smart@...lex.Com>,
	Boaz Harrosh <bharrosh@...asas.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>
Subject: Re: deterministic scsi order with async scan

On Thu, 16 Jul 2009, James Bottomley wrote:

> On Thu, 2009-07-16 at 11:43 -0700, david@...g.hm wrote:
>> On Thu, 16 Jul 2009, James Smart wrote:
>>
>>> david@...g.hm wrote:
>>>> On Thu, 16 Jul 2009, Boaz Harrosh wrote:
>>>>
>>>>
>>>>> It is highly discouraged to setup any kind of system that depends
>>>>> on device-names for block-devices. mounts have the mount by-label
>>>>> or mount by-uuid. Any other subsystem should go by /dev/disk/by-id/*
>>>>> slinks to find a persistent raw block-device. the id is generated
>>>>> from characteristics inside the disk itself so it will be the same
>>>>> no matter what host connection or bus it is connected too (almost).
>>>>>
>>>>> This is because even if the boot order is consistent, the device-name
>>>>> is so volatile in the life-span of a system. Did I boot with a removable
>>>>> USB inserted. that camera or printer was on or off, disk was connected
>>>>> to the other port. Any such change will break things and give you a very
>>>>> poor user experience.
>>>>>
>>>>
>>>> for a laptop you areprobably correct, but for a server or embedded system
>>>> that doesn't have it's hardware changing all the time you are not correct.
>>>>
>>>> especially on a system with lots of drives, why should I have to create an
>>>> initrd that goes and searches dozens or hundreds of drives to find out
>>>> which one to boot from?
>>>>
>>> Boaz is correct. Many enterprise SCSI subsystems (FC, SAS) do not have hard
>>> transport addresses for each device like Parallel SCSI used to.  Thus, any
>>> difference in order of appearance of the devices (power-up ordering, FC ALPA
>>> assignment based on who's loop master, order that switch reports them, is an
>>> array in a failover mode with 1 controller non-existent), or if LUN
>>> configuration on an array changes, or as a drive may fail (especially with
>>> hundreds), there's no guarantee you will see the same thing in the same order
>>> w/o name binding. Same thing is true if one of those adapters fails or is
>>> swapped out.
>>
>> yes, but does your system change the order of your internal direct
>> attached drives with your FC/SAN drives?
>
> Certainly, it can.  The way BIOS booting gets around this is either to
> use some type of physical indicator (like phy number for SAS) to find C:
> or to use a persistent ID mapping scheme (which is pretty much
> equivalent to our /dev/disk/by-id/ udev one).

so if I don't use udev but do want the async detection my only option to 
have it boot from card 1 instead of card 2 is to just keep rebooting the 
machine until it guesses right?

David Lang
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