Hi Janek, this 'ceph features' output that you're probably referring to (or maybe 'ceph tell mon.X sessions') comes up every now and then, for example [0], [1], [2]. I also still don't have a definitive answer, unfortunately. But from my understanding those are compatibility features that don't necessarily reflect the actual ceph version. For example, I have a test cluster with tentacle: tentacle:~ # ceph versions { "mon": { "ceph version 20.2.0 (69f84cc2651aa259a15bc192ddaabd3baba07489) tentacle (stable - RelWithDebInfo)": 1 }, "mgr": { "ceph version 20.2.0 (69f84cc2651aa259a15bc192ddaabd3baba07489) tentacle (stable - RelWithDebInfo)": 2 }, "osd": { "ceph version 20.2.0 (69f84cc2651aa259a15bc192ddaabd3baba07489) tentacle (stable - RelWithDebInfo)": 3 }, "mds": { "ceph version 20.2.0 (69f84cc2651aa259a15bc192ddaabd3baba07489) tentacle (stable - RelWithDebInfo)": 1 }, "overall": { "ceph version 20.2.0 (69f84cc2651aa259a15bc192ddaabd3baba07489) tentacle (stable - RelWithDebInfo)": 7 } } but 'ceph features' shows squid: tentacle:~ # ceph features { "mon": [ { "features": "0x3f07fffffffdffff", "release": "squid", "num": 1 } ], "mds": [ { "features": "0x3f07fffffffdffff", "release": "squid", "num": 1 } ], "osd": [ { "features": "0x3f07fffffffdffff", "release": "squid", "num": 3 } ], ... And this cluster also has "luminous" clients connected although I enforced "squid" with 'ceph osd set-require-min-compat-client squid'. In our production cluster I haven't enforced that, it is still running with luminous as min-compat-client because we actually do have a couple of older clients and don't want to risk anything, but we're going to have to deal with them at some point. Anyway, I'm not recommending to enforce the require-min-compat-client update, I'd rather suggest to set up a test cluster with a couple of your typical clients maybe, and then see if they successfully connect. I did the same in my lab virtual environment that resembles our prod. cluster (OpenStack with Ceph), there I enforced "reef" as min-compat-client and new mounts from Ubuntu machines (24.04, kernel 6.8.0) with Ceph 19.2.1 packages still report luminous on a CephFS mount. But the mount works, so I assume that everything is okay in the lab. But I'm still not feeling confident enough to recommend you to update it. ;-) Hopefully someone else chimes in and contradicts or confirms my statement(s). Regards, Eugen [0] https://lists.ceph.io/hyperkitty/list/ceph-users@ceph.io/thread/A4IWXINDBM6K... [1] https://lists.ceph.io/hyperkitty/list/ceph-users@ceph.io/thread/RVND3Y3Z7FDR... [2] https://lists.ceph.io/hyperkitty/list/ceph-users@ceph.io/thread/WSADGYBR7PLU... Am Mi., 28. Jan. 2026 um 09:16 Uhr schrieb Janek Bevendorff via ceph-users < ceph-users@ceph.io>:

I just noticed that this applies to RBD clients as well. How is this supposed to work? Can you use only radosgw and ceph-fuse if you want to use upmap-read? It seems strange to me that the most central Ceph systems don't work with these new features.

On 26/01/2026 18:55, Janek Bevendorff via ceph-users wrote:

...

Hi,

Is there a way to use the new upmap-read balancer mode when I have CephFS kernel clients connected? Our cluster runs Squid and all connected clients are on kernel 6.11-6.14, so they should be new enough. However, they identify themselves as luminous (I could not find clear documentation about this, but this seems to be hard-coded and by design), so I cannot run ceph osd set-require-min-compat-client reef, which is required for the balancer module to work.

I have 500 clients connected and migrating them all to FUSE would be a headache. Is there a way around that? Some of my pools have extremely high read balance scores and we're also struggling with performance issues as a result.

Janek

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