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What kind of performance can I expect from a Raspberry Pi 2 as a NAS with 2 External hard-drives as a software RAID-1?

I know that the Ethernet runs as a USB device so they all (Ethernet and 2 external HDDs) share a single USB bus.

I already have a setup like this and am wondering if I'm seeing the full potential when I run tests.

I have achieved the following through tests:

SETUP:

Raspberry Pi 2 overclocked to “Medium” via raspi-config, 2 x Seagate Expansion 3TBytes external hard-drives

Software RAID level 1 setup using mdadm:

mdadm -Cv /dev/md1 -l1 -n2 /dev/sd[ab]2

File-system ext4 type created with:

mkfs /dev/md0 -t ext4

Performance test against a Desktop PC running Ubuntu 12.04, connected with wired network.

Using iperf, Raspberry Pi as server:

$ iperf -c raspberrypi -t30
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-30.0 sec   300 MBytes  83.8 Mbits/sec

Write onto software RAID file system using ddon the Raspberry Pi

$ dd if=/dev/zero of=testfile6 bs=4M count=75
314572800 bytes (315 MB) copied, 17.2022 s, 18.3 MB/s

Copy from/to software RAID file system using dd on raspberry pi

$ dd if=testfile6 of=testfile7 bs=4M
314572800 bytes (315 MB) copied, 35.2339 s, 8.9 MB/s

I then exported the RAID drive as a NFS and made write tests on the Desktop PC after having mounted the NFS. Again, using dd:

$ dd if=/dev/zero of=testfile8 bs=4M count=75
314572800 bytes (315 MB) copied, 50,9384 s, 6,2 MB/s

And a read test using dd:

$ dd if=testfile9 of=/dev/null bs=4M
314572800 bytes (315 MB) copied, 57,3728 s, 5,5 MB/s

I guess that writing to the software RAID over NFS require writing data to both external HDDs and an equal bandwidth share goes to the Ethernet controller.

However, if I can write directly to the software RAID with a speed of 18.3 MB/s which is the speed of writing all data twice, then I would expect an actual bandwidth of double that, i.e. 36.6 MB/s.

This bandwidth should be divided by 3 when data comes from the Ethernet controller meaning I should be able to transfer at (36.6 / 3) MB/s = 12.2 MB/s.

The iperf test showed that the Ethernet controller is able to transfer at 83.8 MBits/s = 10.4 MB/s so I guess this is the bottleneck, but then, why do I only get around 6 MB/s?

  • If you want legitmate times not influenced by the file cache you should sudo sync after writing a file and drop the caches before reading one (echo 3 > /proc/sys/vm/drop_caches, but you must do that su root, sudo will not work). – goldilocks Jan 17 '16 at 20:34
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    Not like the RPi has much ram for file caches anyways... to drop the caches easily, try this: echo 3 | sudo tee /proc/sys/vm/drop_caches >/dev/null, no su required. – lornix Jan 17 '16 at 21:10

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