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I have just set up mdadm for RAID0 with two 5400rpm 2.5'' HDDs.

By performing a write speed test using dd:

dd if=/dev/zero of=./largefile bs=1M count=3000

I get:

3000+0 records in

3000+0 records out

3145728000 bytes (3.1 GB, 2.9GiB) copied, 0.986443 s, 3.2 GB/s

which obviously doesn't make sense - tested with copying and opening files, the drive does work all right though.

Anyone knows what's going on here?

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Use the oflag=sync option and you'll see relevant speeds:

dd if=/dev/zero of=./largefile bs=1M count=3000 oflag=sync

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Just as a comparison for my single NMVe M.2 SSD:

$ dd if=/dev/zero of=./largefile bs=1M count=3000
3000+0 records in
3000+0 records out
3145728000 bytes (3.1 GB, 2.9 GiB) copied, 1.98838 s, 1.6 GB/s

$ dd if=/dev/zero of=./largefile bs=1M count=3000 oflag=sync
3000+0 records in
3000+0 records out
3145728000 bytes (3.1 GB, 2.9 GiB) copied, 23.9874 s, 131 MB/s

$ time cp -a ./largefile ./largefile2

real    0m4.738s
user    0m0.021s
sys     0m2.811s

The file is 3 GB / 4.738 seconds = 633 MB/sec. So dd is wrong measuring stick in each test scenario. I would use cp as the real world test.

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