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List:       ceph-users
Subject:    [ceph-users] Re: High ceph_osd_commit_latency_ms on Toshiba MG07ACA14TE HDDs
From:       Paul Emmerich <paul.emmerich () croit ! io>
Date:       2020-06-24 15:30:51
Message-ID: CAD9yTbHnVXsSQBbgT9hbxyTC0cULnoeU2fHuEFg1YmuUKN4pmg () mail ! gmail ! com
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Has anyone ever encountered a drive with a write cache that actually
*helped*?
I haven't.

As in: would it be a good idea for the OSD to just disable the write cache
on startup? Worst case it doesn't do anything, best case it improves
latency.

Paul

-- 
Paul Emmerich

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On Wed, Jun 24, 2020 at 3:49 PM Frank R <frankaritchie@gmail.com> wrote:

> fyi, there is an interesting note on disabling the write cache here:
> 
> 
> https://yourcmc.ru/wiki/index.php?title=Ceph_performance&mobileaction=toggle_view_desktop#Drive_cache_is_slowing_you_down
>  
> On Wed, Jun 24, 2020 at 9:45 AM Benoît Knecht <bknecht@protonmail.ch>
> wrote:
> > 
> > Hi Igor,
> > 
> > Igor Fedotov wrote:
> > > for the sake of completeness one more experiment please if possible:
> > > 
> > > turn off write cache for HGST drives and measure commit latency once
> again.
> > 
> > I just did the same experiment with HGST drives, and disabling the write
> cache
> > on those drives brought the latency down from about 7.5ms to about 4ms.
> > 
> > So it seems disabling the write cache across the board would be
> advisable in
> > our case. Is it recommended in general, or specifically when the DB+WAL
> is on
> > the same hard drive?
> > 
> > Stefan, Mark, are you disabling the write cache on your HDDs by default?
> > 
> > Cheers,
> > 
> > --
> > Ben
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