--As of February 19, 2011 12:01:37 PM +0000, Matthew Seaman is alleged to
have said:
Let's say I install a FreeBSD system using a ZFS-only filesystem into a
box with hotswapable hard drives, configured with some redundancy. Time
passes, one of the drives fails, and it is replaced and rebuilt using the
ZFS tools. (Possibly on auto, or possibly by just doing a 'zpool
replace'.)
Is that box still bootable? (It's still running, but could it *boot*?)
Why wouldn't it be? The configuration in the Wiki article sets aside a
small freebsd-boot partition on each drive, and the instructions tell
you to install boot blocks as part of that partitioning process. You
would have to repeat those steps when you install your replacement drive
before you added the new disk into your zpool.
So long as the BIOS can read the bootcode from one or other drives, and
can then access /boot/zfs/zpool.cache to learn about what zpools you
have, then the system should boot.
So, assuming a forgetful sysadmin (or someone who is new didn't know about
the setup in the first place) is that a yes or a no for the one-drive
replaced case?
It definitely is a 'no' for the all-drives replaced case, as I suspected:
You would need to have repeated the partitioning manually. (And not
letting ZFS handle it.)
If not, what's the minimum needed to support booting from another disk,
and using the ZFS filesystem for everything else?
This situation is described in the Boot ZFS system from UFS article
here: http://wiki.freebsd.org/RootOnZFS/UFSBoot
I use this sort of setup for one system where the zpool has too many
drives in it for the BIOS to cope with; works very well booting from a
USB key.
Thanks; I wasn't sure if that procedure would work if the bootloader was on
a different physical disk than the rest of the filesystem. Nice to hear
from someone who's tried it that it works. ;)
In fact, while the partitioning layout described in the
http://wiki.freebsd.org/RootOnZFS articles is great for holding the OS
and making it bootable, for using ZFS to manage serious quantities of
disk storage, other strategies might be better. It would probably be a
good idea to have two zpools: one for the bulk of the space built from
whole disks (ie. without using gpart or similar partitioning), in
addition to your bootable zroot pool. Quite apart from wringing the
maximum usable space out of your available disks, this also makes it
much easier to replace failed disks or use hot spares.
If a single disk failure in the zpool can render the machine unbootable,
it's better yet to have a dedicated bootloader drive: It increases the mean
time between failures of your boot device (and therefore your machine), and
it reduces the 'gotcha' value. In a hot-swap environment booting directly
off of ZFS you could fail a reboot a month (or more...) after the disk
replacement, and finding your problem then will be a headache until someone
remembers this setup tidbit.
If the 'fail to boot' only happens once *all* the original drives have been
replaced the mean time between failures is better in the ZFS situation, but
the 'gotcha' value becomes absolutely huge: Since you can replace one (or
two, or more) disks without issue, the problem will likely take years to
develop.
Ah well, price of the bleeding edge. ;)
Daniel T. Staal
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