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

Peter Jeremy peter at rulingia.com
Thu Mar 7 07:22:08 UTC 2013

On 2013-Mar-04 16:48:18 -0600, Karl Denninger <karl at denninger.net> wrote:
>The subject machine in question has 12GB of RAM and dual Xeon
>5500-series processors.  It also has an ARECA 1680ix in it with 2GB of
>local cache and the BBU for it.  The ZFS spindles are all exported as
>JBOD drives.  I set up four disks under GPT, have a single freebsd-zfs
>partition added to them, are labeled and the providers are then
>geli-encrypted and added to the pool.

What sort of disks?  SAS or SATA?

>also known good.  I began to get EXTENDED stalls with zero I/O going on,
>some lasting for 30 seconds or so.  The system was not frozen but
>anything that touched I/O would lock until it cleared.  Dedup is off,

When the system has stalled:
- Do you see very low free memory?
- What happens to all the different CPU utilisation figures?  Do they
  all go to zero?  Do you get high system or interrupt CPU (including
  going to 1 core's worth)?
- What happens to interrupt load?  Do you see any disk controller

Would you be able to build a kernel with WITNESS (and WITNESS_SKIPSPIN)
and see if you get any errors when stalls happen.

On 2013-Mar-05 14:09:36 -0800, Jeremy Chadwick <jdc at koitsu.org> wrote:
>On Tue, Mar 05, 2013 at 01:09:41PM +0200, Andriy Gapon wrote:
>> Completely unrelated to the main thread:
>> on 05/03/2013 07:32 Jeremy Chadwick said the following:
>> > That said, I still do not recommend ZFS for a root filesystem
>> Why?
>Too long a history of problems with it and weird edge cases (keep
>reading); the last thing an administrator wants to deal with is a system
>where the root filesystem won't mount/can't be used.  It makes
>recovery or problem-solving (i.e. the server is not physically accessible
>given geographic distances) very difficult.

I've had lots of problems with a gmirrored UFS root as well.  The
biggest issue is that gmirror has no audit functionality so you
can't verify that both sides of a mirror really do have the same data.

>My point/opinion: UFS for a root filesystem is guaranteed to work
>without any fiddling about and, barring drive failures or controller
>issues, is (again, my opinion) a lot more risk-free than ZFS-on-root.

AFAIK, you can't boot from anything other than a single disk (ie no

Peter Jeremy
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