[Gluster-users] Retraction: Protocol stacking: gluster over NFS

[harry mangalam]

Hi All,

Well, it <http://goo.gl/hzxyw> was too good to be true. Under extreme, 
extended IO on a 48core node, some part of the the NFS stack collapses and 
leads to an IO lockup thru NFS.  We've replicated it on 48core and 64 core 
nodes, but don't know yet whether it acts similarly on lower-core-count nodes.

Tho I haven't had time to figure out exactly /how/ it collapses, I owe it to 
those who might be thinking of using it to tell them not to.

This is what I wrote, describing the situation to some co-workers:
===============
With Joseph's and Kevin's help, I've been able to replicate Kevin's complete 
workflow on BDUC and executed it with a normally mounted gluster fs and my 
gluster-via-NFS-loopback (on both NFS3 and NFS4 clients).  

The good news is that the workflow went to completion on BDUC with the native 
gluster fs mount, doing pretty decent IO on one node - topping out at about 
250MB/s in and 75MB/s out (DDR IB)
        ib1        
  KB/s in  KB/s out
 268248.1  62278.40
 262835.1  64813.55
 248466.0  61000.24
 250071.3  67770.03
 252924.1  67235.13
 196261.3  56165.20
 255562.3  68524.45
 237479.3  68813.99
 209901.8  73147.73
 217020.4  70855.45

The bad news is that I've been able to replicate the failures that JF has 
seen.  The workflow starts normally but then eats up free RAM as KT's workflow 
saturates the nodes with about 26 instances of samtools which does a LOT of 
IO (10s of GB in the ~30m of the run).  This was the case even when I 
increased the number of nfsd's to 16 and even 32.

When using native gluster, the workflow goes to completion in about 23 hrs - 
about the same as when KT executed it on his machine (using NFS I think..?).
However when using the loopback mount, on both NFS3 and NFS4, it locks up the 
NFS side (the gluster mount continues to be R/W), requiring a hard reset on 
the node to clear the NFS error.  It is interesting that the samtools 
processes lock up during /reads/, not writes (via stracing several of the 
processes)

I found this entry in a FraunhoferFS discussion:
from <https://groups.google.com/forum/?fromgroups=#!topic/fhgfs-
user/XoGPbv3kfhc>
[[
In general, any network file system that uses the standard kernel page 
cache on the client side (including e.g. NFS, just to give another 
example) is not suitable for running client and server on the same 
machine, because that would lead to memory allocation deadlocks under 
high memory pressure - so you might want to watch out for that. 
(fhgfs uses a different caching mechanism on the clients to allow 
running it in such scenarios.) 
]]
but why this would be the case, I'm not sure - the server and client processes 
should be unable to step on each others data structures, so why they would 
interfere with each other is unclear.  Others on this list have mentioned 
similar opinions - I'd be interested in why this is theoretically the case.

The upshot is that under extreme, extended IO, NFS will lock up, so while we 
haven't seen it on BDUC except for KT's workflow, it's repeatable and we can't 
recover from it smoothly.  So we should move away from it.

I haven't been able to test it on a 3.x kernel (but will after this weekend); 
it's possible that it might work better, but I'm not optimistic.

-- 
Harry Mangalam - Research Computing, OIT, Rm 225 MSTB, UC Irvine
[m/c 2225] / 92697 Google Voice Multiplexer: (949) 478-4487
415 South Circle View Dr, Irvine, CA, 92697 [shipping]
MSTB Lat/Long: (33.642025,-117.844414) (paste into Google Maps)
--
Passive-Aggressive Supporter of the The Canada Party:
  <http://www.americabutbetter.com/>