Hi All, I know there has been lots of discussions around needing fast CPU's to get the most out of SSD's. However I have never really ever seen an solid numbers to make a comparison about how much difference a faster CPU makes and if Ceph scales linearly with clockspeed. So I did a little experiment today.
I setup a 1 OSD Ceph instance on a Desktop PC. The Desktop has a i5 Sandbybridge CPU with the CPU turbo overclocked to 4.3ghz. By using the userspace governor in Linux, I was able to set static clock speeds to see the possible performance effects on Ceph. My pc only has an old X25M-G2 SSD, so I had to limit the IO testing to 4kb QD=1, as otherwise the SSD ran out of puff when I got to the higher clock speeds. CPU Mhz 4Kb Write IO Min Latency (us) Avg Latency (us) CPU usr CPU sys 1600 797 886 1250 10.14 2.35 2000 815 746 1222 8.45 1.82 2400 1161 630 857 9.5 1.6 2800 1227 549 812 8.74 1.24 3300 1320 482 755 7.87 1.08 4300 1548 437 644 7.72 0.9 The figures show a fairly linear trend right through the clock range and clearly shows the importance of having fast CPU's (Ghz not cores) if you want to achieve high IO, especially at low queue depths. Things to Note These figures are from a desktop CPU, no doubt Xeons will be slightly faster at the same clock speed I assuming using the userspace governor in this way is a realistic way to simulate different CPU clock speeds? My old SSD is probably skewing the figures slightly I have complete control over the turbo settings and big cooling, many server CPU's will limit the max turbo if multiple cores are under load or get too hot Ceph SSD OSD nodes are probably best with high end E3 CPU's as they have the highest clock speeds HDD's with Journals will probably benefit slightly from higher clock speeds, if the disk isn't the bottleneck (ie small block sequential writes) These numbers are for Replica=1, at 2 or 3 these numbers will be at least half I would imagine I hope someone finds this useful Nick _______________________________________________ ceph-users mailing list ceph-users@lists.ceph.com http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
