I'm currently using both iozone and fio to test my disk performance. However, iozone show much much poor speed than what is shown from fio. (about 1/3)

Now the question for me is which number should I use and why there is such a gap.

The command I used are:

iozone -s 40G -i 0 -i 1 -b /mnt/ext/40Gtest.xls -f /mnt/ext/iofile.ioz -w -Racz
fio --randrepeat=1 --ioengine=libaio --direct=1 --gtod_reduce=1 --name=test --filename=/mnt/ext/testfile --bs=64k --iodepth=64 --size=4G --readwrite=read

I don't know iozone but your fio job has a non-tiny block size, you're telling it to bypass the Linux page cache, the file fio is using is only 4Gbytes big and you are allowing up to 64 I/Os to be in flight at the same time. You would need to at least make sure that your iozone job has equivalents of those - at least the going direct, the blocksize and number of I/Os in flight will have a giant impact.

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