[pve-devel] Ceph Octopus is out!

Thomas Lamprecht t.lamprecht at proxmox.com
Wed Mar 25 12:00:21 CET 2020


On 3/25/20 11:17 AM, Alexandre DERUMIER wrote:
> I'm pretty excited by the new write-around cache policy for librbd  && io scheduler:)
> 
> (better than writeback, no read latency impact :)
> 

Better for high perf. write workloads:
> Writes return immediately under both the write-around and write-back policies,
> unless there are more than rbd cache max dirty unwritten bytes to the storage
> cluster. The write-around policy differs from the write-back policy in that it
> does not attempt to service read requests from the cache, unlike the write-back
> policy, and is therefore faster for high performance write workloads. Under the
> write-through policy, writes return only when the data is on disk on all
> replicas, but reads may come from the cache.
-- https://docs.ceph.com/docs/octopus/rbd/rbd-config-ref/#cache-settings

Not sure if all workloads benefit from that.


> librbd now uses a write-around cache policy be default,
> replacing the previous write-back cache policy default.
> This cache policy allows librbd to immediately complete
> write IOs while they are still in-flight to the OSDs.
> Subsequent flush requests will ensure all in-flight
> write IOs are completed prior to completing. The
> librbd cache policy can be controlled via a new
> “rbd_cache_policy” configuration option.
> 
> librbd now includes a simple IO scheduler which attempts to
> batch together multiple IOs against the same backing RBD
> data block object. The librbd IO scheduler policy can be
> controlled via a new “rbd_io_scheduler” configuration
> option.

Let's see how much this brings in performance :)





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