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SubjectRe: deterministic scsi order with async scan
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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




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