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List:       ceph-users
Subject:    [ceph-users] Re: High ceph_osd_commit_latency_ms on Toshiba MG07ACA14TE HDDs
From:       Benoît_Knecht <bknecht () protonmail ! ch>
Date:       2020-06-24 12:53:29
Message-ID: S9qIQ2pz3C9xdELnactUZ5hjwgeLea1mSG53A6eCl_3CSXybYQmlyIdF3t63y8ZQNvA2PtlUWuZSnHqxdBpgVz9jozRCjbmdYot6MSimTpc= () protonmail ! ch
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Thank you all for your answers, this was really helpful!

Stefan Priebe wrote:
> yes we have the same issues and switched to seagate for those reasons.
> you can fix at least a big part of it by disabling the write cache of
> those drives - generally speaking it seems the toshiba firmware is
> broken.
> I was not able to find a newer one.

Good to know that we're not alone :) I also looked for a newer firmware, to no
avail.

Igor Fedotov wrote:
> Benoit, wondering what are the write cache settings in your case?
>
> And do you see any difference after disabling it if any?

Write cache is enabled on all our OSDs (including the HGST drives that don't
have a latency issue).

To see if disabling write cache on the Toshiba drives would help, I turned it
off on all 12 drives in one of our OSD nodes:

```
for disk in /dev/sd{a..l}; do hdparm -W0 $disk; done
```

and left it on in the remaining nodes. I used `rados bench write` to create
some load on the cluster, and looked at

```
avg by (hostname) (ceph_osd_commit_latency_ms * on (ceph_daemon) group_left (hostname) ceph_osd_metadata)
```

in Prometheus. The hosts with write cache _enabled_ had a commit latency around
145ms, while the host with write cache _disabled_ had a commit latency around
25ms. So it definitely helps!

Mark Nelson wrote:
> This isn't the first time I've seen drive cache cause problematic
> latency issues, and not always from the same manufacturer.
> Unfortunately it seems like you really have to test the drives you
> want to use before deploying them them to make sure you don't run into
> issues.

That's very true! Data sheets and even public benchmarks can be quite
deceiving, and two hard drives that seem to have similar performance profiles
can perform very differently within a Ceph cluster. Lesson learned.

Cheers,

--
Ben
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