Messages in this thread | | | Date | Thu, 16 Jul 2009 12:48:06 -0700 (PDT) | From | david@lang ... | Subject | Re: deterministic scsi order with async scan |
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On Thu, 16 Jul 2009, James Bottomley wrote:
> 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).
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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