Hello, On Mon, 16 May 2016 22:40:47 +0200 (CEST) Wido den Hollander wrote: > > > Op 16 mei 2016 om 7:56 schreef Chris Dunlop <chris@xxxxxxxxxxxx>: > > > > > > Hi, > > > > I'm trying to understand the potential impact on an active cluster of > > increasing pg_num/pgp_num. > > > > The conventional wisdom, as gleaned from the mailing lists and general > > google fu, seems to be to increase pg_num followed by pgp_num, both in > > small increments, to the target size, using "osd max backfills" (and > > perhaps "osd recovery max active"?) to control the rate and thus > > performance impact of data movement. > > > > I'd really like to understand what's going on rather than "cargo > > culting" it. > > > > I'm currently on Hammer, but I'm hoping the answers are broadly > > applicable across all versions for others following the trail. > > > > Why do we have both pg_num and pgp_num? Given the docs say "The pgp_num > > should be equal to the pg_num": under what circumstances might you want > > these different, apart from when actively increasing pg_num first then > > increasing pgp_num to match? (If they're supposed to be always the > > same, why not have a single parameter and do the "increase pg_num, > > then pgp_num" within ceph's internals?) > > > > pg_num is the actual amount of PGs. This you can increase without any > actual data moving. > Yes and no. Increasing the pg_num will split PGs, which causes potentially massive I/O. Also AFAIK that I/O isn't regulated by the various recovery and backfill parameters. That's probably why recent Ceph versions will only let you increase pg_num in smallish increments. Moving data (as in redistributing amongst the OSD based on CRUSH) will indeed not happen until pgp_num is also increased. Regards, Christian -- Christian Balzer Network/Systems Engineer chibi@xxxxxxx Global OnLine Japan/Rakuten Communications http://www.gol.com/ _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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