[ceph-users] Block sizes, small files and bluestore

Hi Cephers, I have a lot of questions after reading a lot about bluestore, the min_alloc_size and the impact of writing small files to it through cepfs with erasure coding. In a setup using VMware ESX with its VMFS6 and its 1 MB block size on an ISCSI-LUN mapped from Ceph RBD (replicated) 1. Wouldnt it be better to reduce RBD block size to 1 MB also? 2. When a file with a size smaller than 4k is written to a filesystem within a virtual machine (and all the layers downwards) will the consumed space be 4k? So does the 4MB block size of RBD combine a lot of small files to one big object? When using Cephfs and erasure coding: 1. I assume using a 4k min_alloc_size_hdd would reduce wasted space, but increases fragmentation as Igor wrote. 2. How is the official way to deal with fragmentation in bluestore? Is there a defrag tool available or planned? From a performance perspective: My cluster runs on good old filestore using nvme journals. I am about to migrate to bluestore. 1. With a MaxIOSize of 512KB in VMware, wouldnt bluestore_prefer_deferred_size_hdd = 524288 give me a filestore like behavior? My aim is to have the write latency like in filestore because we have a lot of databases. 2. Are there any tradeoffs doing this? Regards, Dennis