OK, now I understand. Yes, the cache setting will take effect immediately. Its more about do you trust the disk firmware to apply the change correctly in all situations when production IO is active at the same time (will volatile cache be flushed correctly or not)? I would not and rather change the setting while the OSD is down. During benchmarks on raw disks I just switched cache on and off when I needed. There was nothing running on the disks and the fio benchmark is destructive any ways. Best regards, ================= Frank Schilder AIT Risø Campus Bygning 109, rum S14 ________________________________________ From: Sasha Litvak <alexander.v.litvak@xxxxxxxxx> Sent: 21 January 2020 10:19 To: Frank Schilder Cc: ceph-users Subject: Re: low io with enterprise SSDs ceph luminous - can we expect more? [klartext] Frank, Sorry for the confusion. I thought that turning off cache using hdparm -W 0 /dev/sdx takes effect right away and in case of non-raid controllers and Seagate or Micron SSDs I would see a difference starting fio benchmark right after executing hdparm. So I wonder it makes a difference whether cache turned off before OSD started or after. On Tue, Jan 21, 2020, 2:07 AM Frank Schilder <frans@xxxxxx<mailto:frans@xxxxxx>> wrote: > So hdparam -W 0 /dev/sdx doesn't work or it makes no difference? I wrote "We found the raw throughput in fio benchmarks to be very different for write-cache enabled and disabled, exactly as explained in the performance article.", so yes, it makes a huge difference. > Also I am not sure I understand why it should happen before OSD have been started. > At least in my experience hdparam does it to hardware regardless. I'm not sure I understand this question. Ideally it happens at boot time and if this doesn't work, at least sometimes before the OSD is started. Why and how else would one want this to happen? Best regards, ================= Frank Schilder AIT Risø Campus Bygning 109, rum S14 _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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