[Xen-users] xen dom0 nfs hangs, oom killer and more

[Mike <mike+xen@xxxxxxxxxxxxxxxxx>]

Hello,

    I have had long term issues with xen dom0 and NFS / CIFS which 
essentially make it impossible to do any reasonable amount of network 
based filesystem operations from dom0 without the risk of excessive 
blocking or hanging of dom0 processes with messages such as "blocked for 
more than 120 seconds". I last posted about this problem in 2015 and 
documented some very extensive troubleshooting on this problem 
(https://lists.gt.net/xen/users/381469). I have never found a 
resolution, across generations of hardware, OS installs, networks and 
more, the penalty simply seems that if you attempt to use NFS / CIFS 
from xen dom0, you can expect io performance issues akin to dialup 
speeds, and hanging / blocked processes starved for I/O that can only be 
resolved with a reboot of the box.
    So here it is in 2017 and I have taken yet another crack at all 
this. I now have yet again, new network, new servers, new os installs, 
new switches, the works. Not only can you still not use NFS/CIFS in dom0 
without the hangs, new symptoms arise now with OOM killer starting up 
and killing seemingly random processes in dom0 all the while with many 
many gigabytes of free ram, no obvious explanation. I am not even sure 
really how to best document this problem but it's like one of those 
emperor has no clothes things, nfs/cifs is unsafe and brings down xen 
dom0 no special effort required other than simply trying to use these, 
end of story.
    I use NFS and CIFS in non-xen setups (bare metal servers) and find 
both to be reasonably reliable and well performing and for nearly the 
same exact workloads that would be under xen dom0 (backups - to create 
compressed tar files of virtual machine image files for example), so I'm 
a little confident that linux and these filesystems together are 
reasonably stable. What I'd like to know, is if there is anyone else who 
uses nfs/cifs from dom0 and are you successful and what your setup looks 
like?
    I plan on setting up a test soon to smoke this out further. Going 
to have a host set up where I can boot xen or non-xen and run the same 
operations and see if I can show definitely this shows up only under xen 
dom0, and then maybe get a clearer picture of why.
Thank you.


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