The recommendations for cephfs is to make a replicated default data pool, and adding any EC data pools using layouts: https://docs.ceph.com/en/latest/cephfs/createfs/
If erasure-coded pools are planned for file system data, it is best to configure the default as a replicated pool to improve small-object write and read performance when updating backtraces.
I have an cephfs that unfortunately wasn't set up like this: they just made an EC pool on the slow HDDs as the default, which sounds like the worst case scenario to me. I would like to add a NVME data pool to this ceph fs, but recommended gives me pause on if i should instead go through the hassle of creating a new cephfs and migrating all users. I've tried to run some mdtest with small 1k files to see if i could measure this difference, but speed is about the same in my relatively small tests so far. I'm also not sure what impact I should realistically expect here. I don't even know if creating files counts as "updating backtraces", so my testing might just be pointless. I guess my core question is; just how important is this suggestion to keep the default data pool on replicated NVME? Setup: 14 hosts x 42 HDD + 3 NVMEs for db/wal 2*2x25 GbitE bonds 12 hosts x 10 NVME. 2*2x100 GbitE bonds Old CephFS setup: - metadata: replicated NVME - data-pools: EC 10+2 on HDD (i plan to add a EC NVME pool here via layouts) New CephFS setup as recommended: - metadata: replicated NVME - data-pools: replicated NVME (default), EC 8+2 on HDD via layout, EC 8+2 on NVME via layout. Ceph 18.2.7 Best regards, Mikael