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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 15:39:16
Message-ID: CAD9yTbGVDwVAg-Teubc8kEX40vsGRwd19QZc9fqkHdxdsTENNQ () mail ! gmail ! com
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Well, what I was saying was "does it hurt to unconditionally run hdparm -W
0 on all disks?"

Which disk would suffer from this? I haven't seen any disk where this would
be a bad idea


Paul

-- 
Paul Emmerich

Looking for help with your Ceph cluster? Contact us at https://croit.io

croit GmbH
Freseniusstr. 31h
81247 München
www.croit.io
Tel: +49 89 1896585 90


On Wed, Jun 24, 2020 at 5:35 PM Frank Schilder <frans@dtu.dk> wrote:

> Yes, non-volatile write cache helps as described in the wiki. When you
> disable write cache with hdparm, it actually only disables volatile write
> cache. That's why SSDs with power loss protection are recommended for ceph.
> 
> A SAS/SATA SSD without any write cache will perform poorly no matter what.
> 
> Best regards,
> =================
> Frank Schilder
> AIT Risø Campus
> Bygning 109, rum S14
> 
> ________________________________________
> From: Paul Emmerich <paul.emmerich@croit.io>
> Sent: 24 June 2020 17:30:51
> To: Frank R
> Cc: Benoît Knecht; s.priebe@profihost.ag; ceph-users@ceph.io
> Subject: [ceph-users] Re: High ceph_osd_commit_latency_ms on Toshiba
> MG07ACA14TE HDDs
> 
> 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
> 
> Looking for help with your Ceph cluster? Contact us at https://croit.io
> 
> croit GmbH
> Freseniusstr. 31h
> 81247 München
> www.croit.io
> Tel: +49 89 1896585 90
> 
> 
> 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
> > > _______________________________________________
> > > ceph-users mailing list -- ceph-users@ceph.io
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> > 
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