[lustre-discuss] RDMA too fragmented, OSTs unavailable (permanently)

Thomas Roth t.roth at gsi.de
Sat Sep 10 00:38:37 PDT 2016

Hi all,

we are running Lustre 2.5.3 on Infiniband. We have massive problems with clients being unable to communicate with any number of OSTs, rendering the 
entire cluster quite unusable.

Clients show
  > LNetError: 1399:0:(o2iblnd_cb.c:1140:kiblnd_init_rdma()) RDMA too fragmented for at o2ib1 (256): 231/256 src 231/256 dst frags
  > LNetError: 1399:0:(o2iblnd_cb.c:1690:kiblnd_reply()) Can't setup rdma for GET from at o2ib1: -90

which eventually results in OSTs at that nid becoming "temporarily unavailable".
However, the OSTs are never recovered, until they are manually evicted or the host rebooted.

On the OSS side, this reads
  >  LNetError: 13660:0:(o2iblnd_cb.c:3075:kiblnd_check_conns()) Timed out RDMA with at o2ib1 (56): c: 7, oc: 0, rc: 7

We have checked the IB fabric, which shows no errors. Since we are not able to reproduce this effect in a simple way, we have also scrutinized the 
user code, so far without results.

Whenever this happens, the connection between client and OSS is fine under all IB test commands.
Communication between client and OSS is still going on, but obviously when Lustre tries to replay the missed transaction, this fragmentation limit is 
hit again, so the OST never becomes available again.

If we understand correctly, the map_on_demand parameter should be increased as a workaround.
The ko2iblnd module seems to provide this parameter,
 > modinfo ko2iblnd
 > parm:           map_on_demand:map on demand (int)

but no matter what we load the module with, map_on_demand always remains at the default value,
 > cat /sys/module/ko2iblnd/parameters/map_on_demand
 > 0

Is there any way to understand
- why this memory fragmentation occurs/becomes so large?
- how to measure the real fragmentation degree (o2iblnd simply stops at 256, perhaps we are at 1000?)
- why map_on_demand cannot be changed?

Of course this all looks very much like LU-5718, but our clients are not behind LNET routers.

There is one router which connects to the campus network but is not in use. And there are some routers which connect to an older cluster, but of 
course the old (1.8) clients never show any of these errors.


Thomas Roth
Department: HPC
Location: SB3 1.262
Phone: +49-6159-71 1453  Fax: +49-6159-71 2986

GSI Helmholtzzentrum für Schwerionenforschung GmbH
Planckstraße 1
64291 Darmstadt

Gesellschaft mit beschränkter Haftung
Sitz der Gesellschaft: Darmstadt
Handelsregister: Amtsgericht Darmstadt, HRB 1528

Geschäftsführung: Professor Dr. Karlheinz Langanke
Ursula Weyrich
Jörg Blaurock

Vorsitzender des Aufsichtsrates: St Dr. Georg Schütte
Stellvertreter: Ministerialdirigent Dr. Rolf Bernhardt

More information about the lustre-discuss mailing list