[zfs-discuss] Re: Very newbie question

strntydog at gmail.com strntydog at gmail.com
Thu Oct 24 02:05:43 EDT 2013

>>> 1. ... system that has *only* ZFS drives. 

The system will be very stable and run fine after boot.  The problem isnt 
ZFS per-se. but grub2's ZFS implementation, which is not the same as ZoL 
and is not as reliable, or as well tested or as feature complete.

I can personally attest that during zfs upgrades or grub2 upgrades its 
always a "cross the fingers" scenario for me when rebooting.  Sometimes it 
works fine, sometimes I am  hosed and trying to get it booting using grub 
command line.
It sub optimal, but what i now do is put a cheap usb thumb drive in a usb 
holes and use it for /boot with ext2.  It should be possible to use 2 thumb 
drives and have a /boot and /boot2 if you really need mirroring on it.  I 
only do this so that when i feel comfortable with gub2 stability i can just 
go back to native zfs boot.  Its painless and works reliably.

For me, some or all of this may relate to 2 below but i don't know for sure.

>>> 2. I have read that one should give ZFS the entire drive(s), ,,, What 
penalty does one pay for placing partitions rather than entire drives under 
ZFS control? 

Potentially a very heavy one.  If you layout your partitions in a bad way 
it can totally confuse zfs.  This happened to me and i now have to rebuild 
a server because the pool became unrecoverable after zpool became 
hopelessly confused.  What happens is if you have a partition which ends at 
the end of the drive, and it contains zfs metadata, zfs will not reliably 
be able to determine if the true "whole drive"  (/dev/sda) is the zpool 
device or if its just the last partition (/dev/sda2) which contains that 
metadata.  When that happens on import zpool will fail because it says the 
pool devices are faulted.  I had this happen to two seperate machines, and 
it is the same under ZoL as illumos (which i tested).  A simple 
"workaround"  to force an import is to make a "/dev/disk-by-parts" which 
only has partitions in it and not full devices and then tell zpool to use 
that with '-d'.  That works, but you cant have that at boot, and so things 
get bad when you reboot, it will also probably fail if its a secondary pool 
when zfs tries to 'mount -a' during booting.  If you must make your own 
partitions, make sure the last physical partition on the drive is NOT a zfs 
pool partition.

The 8MB partition Zpool makes when giving it full devices eliminates the 
possibility of Zpool confusing a partition from a whole device containing 
zfs.  I think thats one of its main purposes.

On Wednesday, October 23, 2013 7:33:20 AM UTC+8, D. R. Evans wrote:
> I apologise for the basic nature of this question (or, more accurately, 
> questions). I've read as much as I can find about ZOL, and have become 
> increasingly confused rather than enlightened. 
> 1. I have read that it is either (a) difficult; (b) unreliable, or (c) 
> impossible to have Linux running on a system that has *only* ZFS drives. 
> What's the truth? 
> The basic problem here seems to be the one of booting directly off a ZFS 
> drive. Can this actually be done reliably? If so, where are there some 
> solid 
> instructions? 
> (FYI, I'm currently using Kubuntu.) 
> 2. I have read that one should give ZFS the entire drive(s), not 
> just partitions on the drive(s). But other advice (particularly 
> https://github.com/zfsonlinux/pkg-zfs/wiki/HOWTO-install-Ubuntu-to-a-Native-ZFS-Root-Filesystem) 
> ignores this and tells me to partition the drives so that there's a 
> separate 
> non-ZFS boot partition in addition to a ZFS partition where everything 
> else 
> resides. What penalty does one pay for placing partitions rather than 
> entire 
> drives under ZFS control? 
> The "simple" solution that seems to be most frequently proposed is to use 
> ZFS 
> for everything except /boot. But that doesn't seem to be particularly 
> helpful, 
> since it means that one needs to go through the relative pain of using 
> something like Linux software RAID for the /boot partition. One important 
> reason that I'm looking at ZFS is to try to get away from the relative 
> pain of 
> configuring and managing Linux RAID (I realise there are other very good 
> reasons for preferring ZFS, but if I can't escape Linux RAID completely 
> and I 
> have to use RAID for /boot, then I might as well just stay with with my 
> current functioning ext3-on-RAID configuration). 
> I hope that I'm being sufficiently clear in the midst of my confusion. 
>   Doc Evans 
> -- 
> Web:  http://www.sff.net/people/N7DR 

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