New subject: Reducing RAM usage on production MDS

28 May 2020

On Wed, May 27, 2020 at 10:09 PM Dylan McCulloch &lt;dmc(a)unimelb.edu.au&gt; wrote:
...

 Hi all,

 The single active MDS on one of our Ceph clusters is close to running out of RAM.

 MDS total system RAM = 528GB
 MDS current free system RAM = 4GB
 mds_cache_memory_limit = 451GB
 current mds cache usage = 426GB 
This mds_cache_memory_limit is way too high for the available RAM. We
normally recommend that your RAM be 150% of your cache limit but we
lack data for such large cache sizes.

...
  Presumably we need to reduce our
mds_cache_memory_limit and/or mds_max_caps_per_client, but would like some guidance on
whether it’s possible to do that safely on a live production cluster when the MDS is
already pretty close to running out of RAM.

 Cluster is Luminous - 12.2.12
 Running single active MDS with two standby.
 890 clients
 Mix of kernel client (4.19.86) and ceph-fuse.
 Clients are 12.2.12 (398) and 12.2.13 (3) 
v12.2.12 has the changes necessary to throttle MDS cache size
reduction. You should be able to reduce mds_cache_memory_limit to any
lower value without destabilizing the cluster.

...
  The kernel clients have stayed under
“mds_max_caps_per_client”: “1048576". But the ceph-fuse clients appear to hold very
large numbers according to the ceph-fuse asok.
 e.g.
 “num_caps”: 1007144398,
 “num_caps”: 1150184586,
 “num_caps”: 1502231153,
 “num_caps”: 1714655840,
 “num_caps”: 2022826512, 
This data from the ceph-fuse asok is actually the number of caps ever
received, not the current number. I've created a ticket for this:
https://tracker.ceph.com/issues/45749

Look at the data from `ceph tell mds.foo session ls` instead.

...
  Dropping caches on the clients appears to reduce their
cap usage but does not free up RAM on the MDS. 
The MDS won't free up RAM until the cache memory limit is reached.

...
  What is the safest method to free cache and reduce RAM
usage on the MDS in this situation (without having to evict or remount clients)? 
reduce mds_cache_memory_limit

...
  I’m concerned that reducing mds_cache_memory_limit
even in very small increments may trigger a large recall of caps and overwhelm the MDS.

That used to be the case in older versions of Luminous but not any longer.

-- 
Patrick Donnelly, Ph.D.
He / Him / His
Senior Software Engineer
Red Hat Sunnyvale, CA
GPG: 19F28A586F808C2402351B93C3301A3E258DD79D

Re: Reducing RAM usage on production MDS