The document of ceph osd config(http://docs.ceph.com/docs/master/rados/configuration/osd-config-ref/#caveats) said that using lower shard number may have deleterious effects. I want to know what the deleterious effects are. 

I test the performance of ceph cluster using different osd_op_num_shards and different osd_op_num_threads_per_shard configuration. I found that Ceph cluster will get bad performance if using lower shard number. And I also found that using many thread number with lower shard number can get the same improvement of performance with multi-shard. So I think I can use lower shard number with many thread number per shard to replace many shard number. But I don't know if there are other bad effects besides bad-performance. I want to use dmclock feature in my ceph cluster but it can get good effect only when using lower shard number.

The environment of my ceph cluster:

• 2 nodes
• 3 osds per node
• nvme ssd as osd
• CPU：Intel(R) Xeon(R) Gold 6130
• Mem：187GB

The main configuration of my ceph cluster:

auth cluster required = cephx

auth service required = cephx

auth client required = cephx

osd journal size = 1024

filestore xattr use omap = true

mon_allow_pool_delete=true

objecter_inflight_ops = 10240

objecter_inflight_op_bytes = 104857600

osd_pool_default_size=2

osd_pool_default_min_size=1

osd_op_queue_cut_off=high

osd_op_num_shards_ssd=1

osd_op_num_threads_per_shard_ssd=1

osd_op_queue=wpq

Thanks for assistance!

Liu Chaoyang