Open Stack

Hi Feilong,

 

Before I was able to use the benchmark tool you mentioned, we saw some other slowdowns with Ceph (all flash).  It appears that something must have crashed somewhere since we had to restart a couple things, after which etcd has been performing fine and no more health issues being reported by Magnum.

 

So, it looks like it wasn't etcd related afterall.

 

However, while researching, I found that etcd's fsync on every write (so it guarantees a write cache flush for each write) apparently creates some havoc with some SSDs, where the SSD performs a full cache flush of multiple caches.  This article explains it a LOT better:  https://yourcmc.ru/wiki/Ceph_performance (scroll to the "Drive cache is slowing you down" section)

 

It seems that the optimal configuration for etcd would be to use local drives in each node and be sure that the write cache is disabled in the SSDs - as opposed to using Ceph volumes, which already adds network latency, but can create even more latency for synchronizations due to Ceph's replication.

 

Eric

 

 

From: feilong [mailto:feilong at catalyst.net.nz] 
Sent: Wednesday, January 15, 2020 2:36 PM
To: Eric K. Miller; openstack-discuss at lists.openstack.org
Cc: Spyros Trigazis
Subject: Re: [magnum][kolla] etcd wal sync duration issue

 

Hi Eric,

If you're using SSD, then I think the IO performance should  be OK. You can use this https://github.com/etcd-io/etcd/tree/master/tools/benchmark to verify and confirm that 's the root cause. Meanwhile, you can review the config of etcd cluster deployed by Magnum. I'm not an export of Etcd, so TBH I can't see anything wrong with the config. Most of them are just default configurations.

As for the etcd image, it's built from https://github.com/projectatomic/atomic-system-containers/tree/master/etcd or you can refer CERN's repo https://gitlab.cern.ch/cloud/atomic-system-containers/blob/cern-qa/etcd/

Spyros, any comments?

 

On 14/01/20 10:52 AM, Eric K. Miller wrote:

	Hi Feilong,
	 
	Thanks for responding!  I am, indeed, using the default v3.2.7 version for etcd, which is the only available image.
	 
	I did not try to reproduce with any other driver (we have never used DevStack, honestly, only Kolla-Ansible deployments).  I did see a number of people indicating similar issues with etcd versions in the 3.3.x range, so I didn't think of it being an etcd issue, but then again most issues seem to be a result of people using HDDs and not SSDs, which makes sense.
	 
	Interesting that you saw the same issue, though.  We haven't tried Fedora CoreOS, but I think we would need Train for this.
	 
	Everything I read about etcd indicates that it is extremely latency sensitive, due to the fact that it replicates all changes to all nodes and sends an fsync to Linux each time, so data is always guaranteed to be stored.  I can see this becoming an issue quickly without super-low-latency network and storage.  We are using Ceph-based SSD volumes for the Kubernetes Master node disks, which is extremely fast (likely 10x or better than anything people recommend for etcd), but network latency is always going to be higher with VMs on OpenStack with DVR than bare metal with VLANs due to all of the abstractions.
	 
	Do you know who maintains the etcd images for Magnum here?  Is there an easy way to create a newer image?
	https://hub.docker.com/r/openstackmagnum/etcd/tags/
	 
	Eric
	 
	 
	 
	From: Feilong Wang [mailto:feilong at catalyst.net.nz] 
	Sent: Monday, January 13, 2020 3:39 PM
	To: openstack-discuss at lists.openstack.org
	Subject: Re: [magnum][kolla] etcd wal sync duration issue
	 
	Hi Eric,
	That issue looks familiar for me. There are some questions I'd like to check before answering if you should upgrade to train.
	1. Are using the default v3.2.7 version for etcd?
	2. Did you try to reproduce this with devstack, using Fedora CoreOS driver? The etcd version could be 3.2.26
	I asked above questions because I saw the same error when I used Fedora Atomic with etcd v3.2.7 and I can't reproduce it with Fedora CoreOS + etcd 3.2.26
	 
	 

-- 
Cheers & Best regards,
Feilong Wang (王飞龙)
------------------------------------------------------
Senior Cloud Software Engineer
Tel: +64-48032246
Email: flwang at catalyst.net.nz
Catalyst IT Limited
Level 6, Catalyst House, 150 Willis Street, Wellington
------------------------------------------------------ 
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openstack.org/pipermail/openstack-discuss/attachments/20200116/16ff76db/attachment-0001.html>