Hi guys. I think I've uncovered a bug in the clat_hist log files. My installation of fio uses 1856 bins for the histograms. I am getting all zero in bins with latency greater than 786.92 microseconds. I created traffic conditions that would produce solid latencies above this level, using this input file: [global] name=fcm_sniff_test thread=1 size=100G rw=randread buffer_compress_percentage=${BCP} write_bw_log=qd1read4k write_hist_log=qd1read4k write_iops_log=qd1read4k log_avg_msec=1000 log_hist_msec=1000 disable_lat=1 disable_slat=1 log_unix_epoch=1 random_generator=tausworthe64 randrepeat=0 rwmixread=0 bs=4k direct=1 ioengine=spdk numjobs=1 iodepth=64 time_based=1 runtime=300 [file0] filename=trtype=PCIe traddr=0000.0b.00.0 ns=1 cpumask=0x00000001 Here is a sample line from the log file: 1557941679145, 0, 4096, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 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Meanwhile, the json+ output reports this, starting at about the point of the cutoff: "6520832" : 588, "6586368" : 522, "6651904" : 554, "6717440" : 513, "6782976" : 495, "6848512" : 467, "6914048" : 417, "6979584" : 420, "7045120" : 397, "7110656" : 366, "7176192" : 335, "7241728" : 300, "7307264" : 304, "7372800" : 266, "7438336" : 290, "7503872" : 241, "7569408" : 229, "7634944" : 245, "7700480" : 238, "7766016" : 201, "7831552" : 176, "7897088" : 197, "7962624" : 173, "8028160" : 147, "8093696" : 162, "8159232" : 141, "8224768" : 149, "8290304" : 119, "8355840" : 133, "8454144" : 247, "8585216" : 195, "8716288" : 174, "8847360" : 132, "8978432" : 143, "9109504" : 167, "9240576" : 105, "9371648" : 83, "9502720" : 86, "9633792" : 78, "9764864" : 53, "9895936" : 70, "10027008" : 66, "10158080" : 58, "10289152" : 56, "10420224" : 53, "10551296" : 36, "10682368" : 43, "10813440" : 36, "10944512" : 30, "11075584" : 28, "11206656" : 17, "11337728" : 17, "11468800" : 20, "11599872" : 14, "11730944" : 9, "11862016" : 16, "11993088" : 18, "12124160" : 11, "12255232" : 16, "12386304" : 7, "12517376" : 7, "12648448" : 11, "12779520" : 8, "12910592" : 5, "13041664" : 9, "13172736" : 5, "13303808" : 4, "13434880" : 3, "13565952" : 4, "13697024" : 2, "13828096" : 2, "13959168" : 3, "14090240" : 3, "14221312" : 1, "14352384" : 1, "14614528" : 1, "14876672" : 2, "15007744" : 1, "15138816" : 2, "15269888" : 2, "15400960" : 2, "15663104" : 1, "16187392" : 3, "16318464" : 1, "16711680" : 1, "16908288" : 2, "17170432" : 1, "17694720" : 1, "19267584" : 1 Note that the log file neither includes the tail nor has a perfect numerical match with the json histogram. The json+ output looks reasonable and correct, with a proper tail. I imported that log file into a spreadsheet and aggregated it into a single histogram. There is a high degree of "match" between the log aggregation and the json histogram, but it's not perfect, even below the cutoff. I am running under CentOS 7.5. The command I used to run the job was this: LD_PRELOAD=/home/kingram/tools/spdk/examples/nvme/fio_plugin/fio_plugin BCP=0 fio --output-format=json+ fcm-test >json_output My fio --version output is: [root@gjallahorn 05-qd1read_4k_rand]# fio --version fio-3.13-42-g8066f Thanks very much! Best regards, Kip