Re: deterministic scsi order with async scan

[david]

On Thu, 16 Jul 2009, James Bottomley wrote:

> On Thu, 2009-07-16 at 11:43 -0700, david@xxxxxxx wrote:
> > On Thu, 16 Jul 2009, James Smart wrote:
> >
> > > david@xxxxxxx 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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