ZFS "stalls" -- and maybe we should be talking about defaults?

Daniel Kalchev daniel at digsys.bg
Wed Mar 6 09:59:53 UTC 2013

On 06.03.13 02:42, Steven Hartland wrote:
> ----- Original Message ----- From: "Daniel Kalchev"
>> On Mar 6, 2013, at 12:09 AM, Jeremy Chadwick <jdc at koitsu.org> wrote:
>>> I say that knowing lots of people use ZFS-on-root, which is great -- I
>>> just wonder how many of them have tested all the crazy scenarios and
>>> then tried to boot from things.
>> I have verified that ZFS-on-root works reliably in all of the following
>> scenarios: single disk, one mirror vdev, many mirror vdevs, raidz.
>> Haven't found the time to test many raidz vdevs, I admit. :)
> One thing to watch out for is the available BIOS boot disks. If you try
> to do a large RAIDZ with lots of disk as you root pool your likely to
> run into problems not because of any ZFS issue but simply because the
> disks the BIOS sees and hence tries to boot may not be what you expect.

A prudent system administrator should understand this issue and verify 
that whatever (boot) architecture they come up with, is supported by 
their particular hardware and firmware. This is no different for ZFS 
than for any other case.

The 2nd stage boot from ZFS loader in FreeBSD could in fact end up with 
it's own drive detection code one day, which will eliminate it's 
dependence on BIOS at all. For relatively small systems, where the 
administrator might be careless enough to not consider all scenarios, 
today's BIOSes already provide support for enough devices (e.n. most 
motherboards provide 4-6 SATA ports etc).

Using separate boot pools of just few devices is what I do for large 
storage boxes too. Mostly because I want to be able to fiddle with data 
disks without caring that might impact the OS. Just make sure the BIOS 
does see these in the drives list it creates. That is, don't put the 
boot disks at the last positions in your chassis :) -- use the on-board 
SATA slots that are scanned first -- sadly, almost every vendor provides 
for such drives placed inside the chassis, which makes it very 
inconvenient if one of the drives dies.


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