On Tue, Jul 28, 2026 at 3:05 AM Janne Johansson via ceph-users <ceph-users@ceph.io> wrote:

I think the idea was that if you mount cephfs on a box running OSDs or a mon daemon, and it gets into some kind of trouble like running out of memory or so, then both the client side and the "server" side might be competing for the same resources that aren't there, making it worse.

This is a classic problem in network filesystems: 1) The kernel runs out of memory and needs to flush out buffered writes to free up space 2) The client sends out a network message, which comes back in to the server 3) The server needs to allocate memory to handle the incoming network message 4) ...stuck RBD and CephFS are susceptible to this, if you mount the kernels on the same hosts as the OSDs. But the odds of hitting it are a lot lower on modern systems with their RAM capacities than in the past. -Greg

If you have separate OSD hosts and they are having issues, then cephfs clients will be slow, but not much more than that, and if clients are having resource dips, a separate cluster doesn't mind at all. I think that is where the non-recommendation comes from, in case you do combine them, it will be hard to get out of this situation, any cpu/ram given to the client side would cause more IO to be requested, from the server side that doesn't have it. In any case it might make it lots harder to diagnose which end actually caused the issue if both suffer at the same time.

Den tis 28 juli 2026 kl 14:45 skrev Marek Szuba via ceph-users < ceph-users@ceph.io>:

...

Dear fellow Cephies,

A few weeks ago I came across in Tentacle documentation on docs.ceph.org (and OF COURSE now that I'm trying to quote it, I cannot find it any more) a mention that CephFS should not be mounted on systems serving Ceph because of there being a risk of harmful interference in the kernel. The context suggested this pertained to one-node Ceph clusters, that said I do not believe that was explicitly stated. Therefore, I very much hope someone will be able to enlighten me whether:

* this is indeed still the case with Tentacle,

* it applies to one-node clusters only or all Ceph clusters,

* it only applies to CephFS mounts using the kernel driver, or ceph-fuse as well.

Thank you in advance!

-- MS

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