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@xxxxxxxx> Sent: 24 June 2020 17:30:51 To: Frank R Cc: Benoît Knecht; s.priebe@xxxxxxxxxxxx; ceph-users@xxxxxxx Subject: 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@xxxxxxxxx> 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@xxxxxxxxxxxxx> > 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@xxxxxxx > > To unsubscribe send an email to ceph-users-leave@xxxxxxx > _______________________________________________ > ceph-users mailing list -- ceph-users@xxxxxxx > To unsubscribe send an email to ceph-users-leave@xxxxxxx > _______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx _______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx
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