High Kernel Load with nfsv4

[Rick Macklem]

Loic Blot wrote:
> Hi,
>  i'm trying to create a virtualisation environment based on jails.
>  Those jails are stored under a big ZFS pool on a FreeBSD 9.3 which
>  export a NFSv4 volume. This NFSv4 volume was mounted on a big
>  hypervisor (2 Xeon E5v3 + 128GB memory and 8 ports (but only 1 was
>  used at this time).
> 
>  The problem is simple, my hypervisors runs 6 jails (used 1% cpu and
>  10GB RAM approximatively and less than 1MB bandwidth) and works
>  fine at start but the system slows down and after 2-3 days become
>  unusable. When i look at top command i see 80-100% on system and
>  commands are very very slow. Many process are tagged with nfs_cl*.
> 
To be honest, I would expect the slowness to be because of slow response
from the NFSv4 server, but if you do:
# ps axHl
on a client when it is slow and post that, it would give us some more
information on where the client side processes are sitting.
If you also do something like:
# nfsstat -c -w 1
and let it run for a while, that should show you how many RPCs are
being done and which ones.

# nfsstat -m
will show you what your mount is actually using.
The only mount option I can suggest trying is "rsize=32768,wsize=32768",
since some network environments have difficulties with 64K.

There are a few things you can try on the NFSv4 server side, if it appears
that the clients are generating a large RPC load.
- disabling the DRC cache for TCP by setting vfs.nfsd.cachetcp=0
- If the server is seeing a large write RPC load, then "sync=disabled"
  might help, although it does run a risk of data loss when the server
  crashes.
Then there are a couple of other ZFS related things (I'm not a ZFS guy,
but these have shown up on the mailing lists).
- make sure your volumes are 4K aligned and ashift=12 (in case a drive
  that uses 4K sectors is pretending to be 512byte sectored)
- never run over 70-80% full if write performance is an issue
- use a zil on an SSD with good write performance

The only NFSv4 thing I can tell you is that it is known that ZFS's
algorithm for determining sequential vs random I/O fails for NFSv4
during writing and this can be a performance hit. The only workaround
is to use NFSv3 mounts, since file handle affinity apparently fixes
the problem and this is only done for NFSv3.

rick


>  I saw that there are TSO issues with igb then i'm trying to disable
>  it with sysctl but the situation wasn't solved.
> 
>  Someone has got ideas ? I can give you more informations if you
>  need.
> 
>  Thanks in advance.
>  Regards,
> 
>  Loïc Blot,
>  UNIX Systems, Network and Security Engineer
>  http://www.unix-experience.fr
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