** TL;DR In testing, the write latency performance of a PWL-cache backed RBD disk was 2 orders of magnitude worse than the disk holding the PWL cache. ** Summary I was hoping that PWL cache might be a good solution to the problem of write latency requirements of etcd when running a kubernetes control plane on ceph. Etcd is extremely write latency sensitive and becomes unstable if write latency is too high. The etcd workload can be characterised by very small (~4k) writes with a queue depth of 1. Throughput, even on a busy system, is normally very low. As etcd is distributed and can safely handle the loss of un-flushed data from a single node, a local ssd PWL cache for etcd looked like an ideal solution. My expectation was that adding a PWL cache on a local SSD to an RBD-backed would improve write latency to something approaching the write latency performance of the local SSD. However, in my testing adding a PWL cache to an rbd-backed VM increased write latency by approximately 4x over not using a PWL cache. This was over 100x more than the write latency performance of the underlying SSD. My expectation was based on the documentation here: https://docs.ceph.com/en/quincy/rbd/rbd-persistent-write-log-cache/ “The cache provides two different persistence modes. In persistent-on-write mode, the writes are completed only when they are persisted to the cache device and will be readable after a crash. In persistent-on-flush mode, the writes are completed as soon as it no longer needs the caller’s data buffer to complete the writes, but does not guarantee that writes will be readable after a crash. The data is persisted to the cache device when a flush request is received.” ** Method 2 systems, 1 running single-node Ceph Quincy (17.2.6), the other running libvirt and mounting a VM’s disk with librbd (also 17.2.6) from the first node. All performance testing is from the libvirt system. I tested write latency performance: * Inside the VM without a PWL cache * Of the PWL device directly from the host (direct to filesystem, no VM) * Inside the VM with a PWL cache I am testing with fio. Specifically I am running a containerised test, executed with: podman run --volume .:/var/lib/etcd:Z quay.io/openshift-scale/etcd-perf This container runs: fio --rw=write --ioengine=sync --fdatasync=1 --directory=/var/lib/etcd --size=100m --bs=8000 --name=etcd_perf --output-format=json --runtime=60 --time_based=1 And extracts sync.lat_ns.percentile["99.000000"] ** Results All results were stable across multiple runs within a small margin of error. * rbd no cache: 1417216 ns * pwl cache device: 44288 ns * rbd with pwl cache: 5210112 ns Note that by adding a PWL cache we increase write latency by approximately 4x, which is more than 100x than the underlying device. ** Hardware 2 x Dell R640s, each with Xeon Silver 4216 CPU @ 2.10GHz and 192G RAM Storage under test: 2 x SAMSUNG MZ7KH480HAHQ0D3 SSDs attached to PERC H730P Mini (Embedded) OS installed on rotational disks N.B. Linux incorrectly detects these disks as rotational, which I assume relates to weird behaviour by the PERC controller. I remembered to manually correct this on the ‘client’ machine for the PWL cache, but at OSD configuration time ceph would have detected them as rotational. They are not rotational. ** Ceph Configuration CentOS Stream 9 # ceph version ceph version 17.2.6 (d7ff0d10654d2280e08f1ab989c7cdf3064446a5) quincy (stable) Single node installation with cephadm. 2 OSDs, one on each SSD. 1 pool with size 2 ** Client Configuration Fedora 38 Librbd1-17.2.6-3.fc38.x86_64 PWL cache is XFS filesystem with 4k block size, matching the underlying device. The filesystem uses the whole block device. There is no other load on the system. ** RBD Configuration # rbd config image list libvirt-pool/pwl-test | grep cache rbd_cache true config rbd_cache_block_writes_upfront false config rbd_cache_max_dirty 25165824 config rbd_cache_max_dirty_age 1.000000 config rbd_cache_max_dirty_object 0 config rbd_cache_policy writeback pool rbd_cache_size 33554432 config rbd_cache_target_dirty 16777216 config rbd_cache_writethrough_until_flush true pool rbd_parent_cache_enabled false config rbd_persistent_cache_mode ssd pool rbd_persistent_cache_path /var/lib/libvirt/images/pwl pool rbd_persistent_cache_size 1073741824 config rbd_plugins pwl_cache pool # rbd status libvirt-pool/pwl-test Watchers: watcher=10.1.240.27:0/1406459716 client.14475 cookie=140282423200720 Persistent cache state: host: dell-r640-050 path: /var/lib/libvirt/images/pwl/rbd-pwl.libvirt-pool.37e947fd216b.pool size: 1 GiB mode: ssd stats_timestamp: Mon Jun 26 11:29:21 2023 present: true empty: false clean: true allocated: 180 MiB cached: 135 MiB dirty: 0 B free: 844 MiB hits_full: 1 / 0% hits_partial: 3 / 0% misses: 21952 hit_bytes: 6 KiB / 0% miss_bytes: 349 MiB -- Matthew Booth