[Bug 197164] Zpool with L2ARC hangs whole system

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=197164

            Bug ID: 197164
           Summary: Zpool with L2ARC hangs whole system
           Product: Base System
           Version: 10.1-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: Karli.Sjoberg at slu.se

Created attachment 152328
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=152328&action=edit
Graphite - System Overview

Hi!

At present we have 4 ZFS storage systems that _were_ configured with SSD disks
as cache and after different periods of time, depending on amount of RAM and
load, they go unresponsive.

Initially you can ping them and change VT's at the console but nothing prints
when you type, all services are gone etc. After a while they stop responding to
ping as well. After a reboot all is good again for a while until the process
repeats itself.

Now I have found out exactly what´s causing it: L2ARC! Just removing the cache
drive(s), they run rock-solid again, but performance is severely degraded. The
caching in ZFS really does wonders to offload the "slow" rotating disks and
we´d very much like to be able to re-add them to our pools again.

This is similar to:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=187594

But this might be another issue. And since the OP couldn´t experiment with the
systems being in production, the case couldn´t really come any further, but
this one can! We have a virtual machine set up exactly like our "real"
storage's, but miniturized in performance and capacity. It´s upgraded to
10.1-RELEASE with these patches applied:
https://svnweb.freebsd.org/base?view=revision&revision=272875

With a script that loops copying files from my desktop to the VM and then back
again, I have been able to reliably hang the system just by re-adding the cache
to the pool, take a look at the attached screenshot. It shows the system
overview of this virtual storage server were I was running my script over night
and added the cache to the pool at around 9 AM. See what happens with the ARC?
That´s the problem. And then it went unresponsive around 3-4 PM.

Thanks in advance!
Karli Sjöberg

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