Messages in this thread | | | Date | Thu, 16 Jul 2009 11:43:55 -0700 (PDT) | From | david@lang ... | Subject | Re: deterministic scsi order with async scan |
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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?
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 >
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