On Thu, 2009-07-16 at 11:43 -0700, david@xxxxxxx wrote: > 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? 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 -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
