Re: deterministic scsi order with async scan

[david]

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?

David Lang

> > I am building a system that will have two drives in a hardware mirror on 
> > one SCSI card, and 160 drives on a FC (SCSI) card. why should my boot have 
> > to go and examine all 162 drives to look for an ID on every partition just 
> Because its the only safe way to ensure proper device identification.
>
> -- james s
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/