Official recommendations are for 1 GB of memory for every 1 TB of storage. In truth, there are a number of variables that lead to this recommendation, but suffice to say that you never want to find yourself where your OSDs are running low on memory and any excess memory will be used to improve performance.

Aside from the baseline memory usage of OSD, the main variable effecting memory usage is the number of PGs running on OSD. Although total data size does have an impact on memory usage, it is dwarfed by the effect of the number of PGs. A healthy cluster running within the recommendations of 200 PGs per OSD will probably use less than 2 GB of RAM per OSD. However, in a cluster where the number of PGs has been set higher against best practice, memory usage will be higher. It is also worth noting that when OSD is removed from a cluster, extra PGs will be placed on remaining OSDs to rebalance the cluster; this will also increase memory usage as well as the recovery operation itself. This spike in memory usage can sometimes be the cause of cascading failures if insufficient ram has been provisioned. A large swap partition on SSD should always be provisioned to reduce the risk of the Linux out-of-memory (OOM) killer randomly killing OSD processes in the event of a low memory situation.

As a minimum, look to provision around 2 GB per OSD + OS overheads, but this should be treated as the bare minimum and 4 GB per OSD would be recommended.

Depending on your workload and size of spinning disks being used for the Ceph OSDs, extra memory may be required to ensure that the operating system can sufficiently cache the directory entries and file nodes from the filesystem used to store the Ceph objects. This may have a bearing on the RAM you wish to configure your nodes with and is covered in more detail in the tuning section of the book.

Regardless of the configured memory size, ECC memory should be used at all times.