Inconsistent IO speed with dd command

Solution Verified - Updated -

Issue

  • When I write to the disk and overwriting the file I get significant performance drop:
# time sh -c "dd if=/dev/zero of=ddfile bs=8k count=100000 ; sync"
100000+0 records in
100000+0 records out
819200000 bytes (819 MB) copied, 0.778416 s, 1.1 GB/s

real    0m9.528s
user    0m0.018s
sys     0m0.594s

# time sh -c "dd if=/dev/zero of=ddfile bs=8k count=100000 ; sync"
100000+0 records in
100000+0 records out
819200000 bytes (819 MB) copied, 7.76798 s, 105 MB/s

real    0m9.376s
user    0m0.011s
sys     0m1.084s

Environment

  • Red Hat Enterprise Linux

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