[pve-devel] [PATCH docs] pveceph: update OSD memory considerations

[Aaron Lauterer]

thanks for the patch! see inline for comments

On  2025-09-18  18:45, Alwin Antreich wrote:
> Since bluestore, OSDs adhere to the osd_memory_target and the
> recommended amount of memory was increased.
> 
> See: https://docs.ceph.com/en/reef/start/hardware-recommendations/#ram
> 
> Signed-off-by: Alwin Antreich <alwin at antreich.com>
> ---
>   pveceph.adoc | 16 ++++++++--------
>   1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/pveceph.adoc b/pveceph.adoc
> index 17efa4d..a2d71e7 100644
> --- a/pveceph.adoc
> +++ b/pveceph.adoc
> @@ -131,14 +131,14 @@ carefully planned out and monitored. In addition to the predicted memory usage
>   of virtual machines and containers, you must also account for having enough
>   memory available for Ceph to provide excellent and stable performance.
>   
> -As a rule of thumb, for roughly **1 TiB of data, 1 GiB of memory** will be used
> -by an OSD. While the usage might be less under normal conditions, it will use
> -most during critical operations like recovery, re-balancing or backfilling.
> -That means that you should avoid maxing out your available memory already on
> -normal operation, but rather leave some headroom to cope with outages.
> -
> -The OSD service itself will use additional memory. The Ceph BlueStore backend of
> -the daemon requires by default **3-5 GiB of memory** (adjustable).
> +While usage may be less under normal conditions, it will consume more memory
> +during critical operations, such as recovery, rebalancing, or backfilling. That
> +means you should avoid maxing out your available memory already on regular
> +operation, but rather leave some headroom to cope with outages.
> +
> +The current recommendation is to configure at least **8 GiB of memory per OSD
> +daemon** for good performance. The OSD daemon requires, by default, 4 GiB of
> +memory.

given how the current latest Ceph docs phrase it [0], I am not sure 
here. They sound like the default osd_memory_target of 4G is okay, but 
that they might use more in recovery situations and one should calculate 
with ~8G.

So unless I understand that wrong, maybe we could phrase it more like 
the following?
===
The current recommendation is to calculate with at least 8 GiB of memory 
per OSD daemon to give it enough memory if needed. By default, the OSD 
daemon is set to use up to 4 GiB of memory in normal scenarios.
===

If I understand it wrong and users should change the osd_memory_target 
to 8 GiB, we should document how, or maybe even try to make it 
configurable in the GUI/API/pveceph...



[0] https://docs.ceph.com/en/latest/start/hardware-recommendations/#ram



>   
>   [[pve_ceph_recommendation_network]]
>   .Network