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Re: [zfs-discuss] Mirroring USB Drive with Laptop for Backup purposes

Subject: Re: [zfs-discuss] Mirroring USB Drive with Laptop for Backup purposes
From: Ragnar Sundblad
Date: Fri, 14 May 2010 10:23:22 +0200
On 12 maj 2010, at 22.39, Miles Nordin wrote:

>>>>>> "bh" == Brandon High <[email protected]> writes:
>    bh> If you boot from usb and move your rpool from one port to
>    bh> another, you can't boot. If you plug your boot sata drive into
>    bh> a different port on the motherboard, you can't
>    bh> boot. Apparently if you are missing a device from your rpool
>    bh> mirror, you can't boot.
> yeah, this is retarded and should get fixed.
>    bh> zpool.cache saves the device path to make importing pools
>    bh> faster. It would be nice if there was a boot flag you could
>    bh> give it to ignore the file...
> I've no doubt this is true but ISTR it's not related to the booting
> problem above becuase I do not think zpool.cache is used to find the
> root pool.  It's only used for finding other pools.  ISTR the root
> pool is found through devid's that grub reads from the label on the
> BIOS device it picks, and then passes to the kernel.  note that
> zpool.cache is ON THE POOL, so it can't be used to find the pool (ok,
> it can---on x86 it can be sync'ed into the boot archive, and on SPARC
> it can be read through the PROM---but although I could be wrong ISTR
> this is not what's actually done).
> I think you'll find you CAN move drives among sata ports, just not
> among controller types, because the devid is a blob generated by the
> disk driver, and pci-ide and AHCI will yeild up different devid's for
> the same disk.  Grub never calculates a devid, just reads one from the
> label (reads a devid that some earlier kernel got from pci-ide or ahci
> and wrote into the label).  so when ports and device names change,
> rewriting labels is helpful but not urgent.  When disk drivers change,
> rewriting labels is urgent.

Are you talking about booting here?

Because with the OS booted, the devid should only be a hint, and if it
is found to not be correct the the disks should be found with their
guids by searching all /dev/dsk/* devices, shouldn't they?

So, if everything worked as expected, I don't see any reason at all to
ever have to remove/ignore the zpool.cache. (Well - except for the case
when you don't want the OS to import pools that where imported before
shutdown/crash - but that hasn't been discussed here yet.)

Finding disks at boot is a very different animal and have it's
limitations, though GRUB tries to get work around some of them.

Am I missing something?


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