I'm trying to use flashbench to characterize a Kingston 8 GB data
traveler USB flash drive I have and am getting strange results. Could
someone help me understand how the diff can be negative? It seems that
the drive reads faster when crossing almost all boundaries rather than
slower:
psusi@arafel:/usr/share/doc/flashbench$ sudo flashbench -a
--blocksize=1024 /dev/sdb
align 2147483648 pre 766µs on 737µs post 869µs diff
-80712n
align 1073741824 pre 783µs on 740µs post 850µs diff
-76424n
align 536870912 pre 745µs on 737µs post 852µs diff -61441n
align 268435456 pre 748µs on 736µs post 846µs diff -61366n
align 134217728 pre 753µs on 739µs post 852µs diff -63074n
align 67108864 pre 747µs on 738µs post 849µs diff -59359n
align 33554432 pre 753µs on 739µs post 850µs diff -61974n
align 16777216 pre 764µs on 757µs post 866µs diff -57423n
align 8388608 pre 763µs on 758µs post 861µs diff -53705n
align 4194304 pre 764µs on 739µs post 847µs diff -66497n
align 2097152 pre 749µs on 740µs post 856µs diff -62748n
align 1048576 pre 811µs on 806µs post 902µs diff -50206n
align 524288 pre 809µs on 803µs post 910µs diff -56065n
align 262144 pre 810µs on 806µs post 909µs diff -53535n
align 131072 pre 809µs on 801µs post 909µs diff -57797n
align 65536 pre 811µs on 807µs post 908µs diff -52651n
align 32768 pre 809µs on 805µs post 907µs diff -53486n
align 16384 pre 808µs on 803µs post 862µs diff -31784n
align 8192 pre 808µs on 800µs post 882µs diff -45365n
align 4096 pre 809µs on 804µs post 803µs diff -1767ns
align 2048 pre 809µs on 808µs post 807µs diff -239ns
psusi@arafel:/usr/share/doc/flashbench$ sudo flashbench -a /dev/sdb
align 2147483648 pre 1.35ms on 1.36ms post 1.36ms diff
3.13µs
align 1073741824 pre 1.36ms on 1.35ms post 1.36ms diff
-5105ns
align 536870912 pre 1.35ms on 1.35ms post 1.36ms diff -3044ns
align 268435456 pre 1.36ms on 1.35ms post 1.36ms diff -4602ns
align 134217728 pre 1.36ms on 1.36ms post 1.35ms diff 2.73µs
align 67108864 pre 1.36ms on 1.35ms post 1.36ms diff -4807ns
align 33554432 pre 1.35ms on 1.36ms post 1.36ms diff 1.34µs
align 16777216 pre 1.37ms on 1.36ms post 1.38ms diff -15503n
align 8388608 pre 1.37ms on 1.37ms post 1.37ms diff 1.2µs
align 4194304 pre 1.35ms on 1.35ms post 1.36ms diff -734ns
align 2097152 pre 1.37ms on 1.36ms post 1.35ms diff -1662ns
align 1048576 pre 1.37ms on 1.37ms post 1.37ms diff -2851ns
align 524288 pre 1.37ms on 1.38ms post 1.37ms diff 1.69µs
align 262144 pre 1.37ms on 1.37ms post 1.38ms diff 146ns
align 131072 pre 1.38ms on 1.37ms post 1.37ms diff -3798ns
align 65536 pre 1.38ms on 1.37ms post 1.38ms diff -2426ns
align 32768 pre 1.37ms on 1.37ms post 1.37ms diff -3169ns
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