Re: deterministic scsi order with async scan

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Thu, 2009-07-16 at 11:43 -0700, david@lang.hm wrote:
> On Thu, 16 Jul 2009, James Smart wrote:
> 
> > david@lang.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).

James