Am 11.03.2015 um 11:17 schrieb Nick Fisk: > > >> Hi Nick, >> >> Am 11.03.2015 um 10:52 schrieb Nick Fisk: >>> Hi Stefan, >>> >>> If the majority of your hot data fits on the cache tier you will see >>> quite a marked improvement in read performance >> I don't have writes ;-) just around 5%. 95% are writes. >> >>> and similar write performance >>> (assuming you would have had your hdds backed by SSD journals). >> >> similar write performance of SSD cache tier or HDD "backend" tier? >> >> I'm mainly interested in a writeback mode. > > Writes on Cache tiering are the same speed as a non cache tiering solution > (with SSD journals), if the blocks are in the cache. > > >> >>> However for data that is not in the cache tier you will get 10-20% >>> less read performance and anything up to 10x less write performance. >>> This is because a cache write miss has to read the entire object from >>> the backing store into the cache and then modify it. >>> >>> The read performance degradation will probably be fixed in Hammer with >>> proxy reads, but writes will most likely still be an issue. >> >> Why is writing to the HOT part so slow? >> > > If the object is in the cache tier or currently doesn't exist, then writes > are fast as it just has to write directly to the cache tier SSD's. However > if the object is in the slow tier and you write to it, then its very slow. > This is because it has to read it off the slow tier (~12ms), write it on to > the cache tier(~.5ms) and then update it (~.5ms). Mhm sounds correct. So it's better to stuck with journals instead of using a cache tier. Stefan > > With a non caching solution, you would have just written straight to the > journal (~.5ms) > >> Stefan >> >>> Nick >>> >>> >>>> -----Original Message----- >>>> From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On Behalf >>>> Of Stefan Priebe - Profihost AG >>>> Sent: 11 March 2015 07:27 >>>> To: ceph-users@xxxxxxxxxxxxxx >>>> Subject: Firefly Tiering >>>> >>>> Hi, >>>> >>>> has anybody successfully tested tiering while using firefly? How much >>>> does >>> it >>>> impact performance vs. a normal pool? I mean is there any difference >>>> between a full SSD pool und a tiering SSD pool with SATA Backend? >>>> >>>> Greets, >>>> Stefan >>>> _______________________________________________ >>>> 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 > > > > _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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