On Sat, Feb 16, 2019 at 12:44 PM Oliver Freyermuth <freyermuth@xxxxxxxxxxxxxxxxxx> wrote: > > Dear Cephalopodians, > > in some recent threads on this list, I have read about the "knobs": > > pglog_hardlimit (false by default, available at least with 12.2.11 and 13.2.5) > bdev_enable_discard (false by default, advanced option, no description) > bdev_async_discard (false by default, advanced option, no description) > > I am wondering about the defaults for these settings, and why these settings seem mostly undocumented. > > It seems to me that on SSD / NVMe devices, you would always want to enable discard for significantly increased lifetime, > or run fstrim regularly (which you can't with bluestore since it's a filesystem of its own). From personal experience, > I have already lost two eMMC devices in Android phones early due to trimming not working fine. > Of course, on first generation SSD devices, "discard" may lead to data loss (which for most devices has been fixed with firmware updates, though). > > I would presume that async-discard is also advantageous, since it seems to queue the discards and work on these in bulk later > instead of issuing them immediately (that's what I grasp from the code). > > Additionally, it's unclear to me whether the bdev-discard settings also affect WAL/DB devices, which are very commonly SSD/NVMe devices > in the Bluestore age. > > Concerning the pglog_hardlimit, I read on that list that it's safe and limits maximum memory consumption especially for backfills / during recovery. > So it "sounds" like this is also something that could be on by default. But maybe that is not the case yet to allow downgrades after failed upgrades? This flag will be on by default in nautilus and that's not the case in luminous and mimic to a handle upgrades. > > > > So in the end, my question is: > Is there a reason why these values are not on by default, and are also not really mentioned in the documentation? > Are they just "not ready yet" / unsafe to be on by default, or are the defaults just like that because they have always been at this value, > and defaults will change with the next major release (nautilus)? We can certainly make this more explicit in our documentation. Thanks, Neha > > Cheers, > Oliver > > _______________________________________________ > ceph-users mailing list > ceph-users@xxxxxxxxxxxxxx > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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