Testing setup:
1. NAS with 8 bays with Atom D2700 dual core @ 2.13 GHz (not a QNAP model, but that 'other' brand, wink wink, psst. the 1813+)
2. QNAP TS-870 with Celeron G550 dual core CPU @ 2.6 GHz
3. QNAP TS-870 with i7 3770t quad core CPU @ 2.5 GHz
4. Disks used (always using disk 1-n when testing with 'n' disks):
disk 1-4: Hitachi Deskstar 1TB 7200 rpm
disk 5-7: WD GREEN 1TB
disk 8: WD GREEN 2TB
5. Disable syncing (echo idle > /sys/block/md1/md/sync_action) -- avoid 10% performance hit
6. Disable thin provisioning -- avoid 15% performance hit
I have tested with a very simple dd command on the NAS itself, and I have looked only at the write performance:
After this, I looked at the 'real' time spent (wall clock time elapsed). For instance with 30.00 seconds of 'real' time:time dd if=/dev/zero of=/share/MD0_DATA/homes/admin/blah bs=4M count=2048 conv=sync
I would write down 273.1 MB/s as write speed. All tests were performed at least twice, and if unexpectedly low performance occurred (e.g. measuring 167.0 MB/s after a 273.1 MB/s) then it's fair to make another run and ignore the bad run. That has only happened on very few occasions.4*2048/30.00
273.06666666666666666666
So here we go...
1. NAS with 8 bays with Atom D2700 dual core @ 2.13 GHz
3 disks in raid-5 72.11 MB/s 76.35 MB/s
4 disks in raid-5 167.1 MB/s 176.2 MB/s
5 disks in raid-5 210.5 MB/s 176.2 MB/s
6 disks in raid-5 194.1 MB/s 198.3 MB/s
7 disks in raid-5 199.4 MB/s 215.6 MB/s
8 disks in raid-5 229.4 MB/s 200.6 MB/s
4 disks in raid-6 113.6 MB/s 110.8 MB/s
5 disks in raid-6 154.8 MB/s 135.3 MB/s
6 disks in raid-6 175.8 MB/s 165.3 MB/s
7 disks in raid-6 190.5 MB/s 192.6 MB/s
8 disks in raid-6 196.0 MB/s 177.9 MB/s
4 disks in raid-10 155.2 MB/s 147.7 MB/s
6 disks in raid-10 227.4 MB/s 218.8 MB/s
8 disks in raid-10 238.9 MB/s 249.5 MB/s
2. QNAP TS-870 with Celeron G550 dual core CPU @ 2.6 GHz
3 disks in raid-5 135.3 MB/s 134.8 MB/s
4 disks in raid-5 202.1 MB/s 203.4 MB/s
5 disks in raid-5 275.4 MB/s 268.0 MB/s
6 disks in raid-5 319.8 MB/s 325.3 MB/s
7 disks in raid-5 364.7 MB/s 373.2 MB/s
8 disks in raid-5 356.4 MB/s 382.8 MB/s
4 disks in raid-6 133.9 MB/s 133.3 MB/s
5 disks in raid-6 207.8 MB/s 211.7 MB/s
6 disks in raid-6 270.1 MB/s 271.4 MB/s
7 disks in raid-6 314.6 MB/s 317.6 MB/s crypted: 123.1 MB/s 125.5 MB/s
8 disks in raid-6 338.1 MB/s 353.1 MB/s crypted: 124.5 MB/s 124.7 MB/s
4 disks in raid-10 147.2 MB/s 150.4 MB/s
6 disks in raid-10 229.3 MB/s 234.8 MB/s
8 disks in raid-10 295.2 MB/s 298.6 MB/s
3. QNAP TS-870 with i7 3770t quad core CPU @ 2.5 GHz
3 disks in raid-5 155.6 MB/s 156.1 MB/s
4 disks in raid-5 241.8 MB/s 230.9 MB/s
5 disks in raid-5 325.2 MB/s 316.5 MB/s
6 disks in raid-5 403.8 MB/s 364.8 MB/s
7 disks in raid-5 479.0 MB/s 434.9 MB/s
8 disks in raid-5 497.6 MB/s 491.0 MB/s crypted: 424.5 MB/s 400.1 MB/s
4 disks in raid-6 155.2 MB/s 159.5 MB/s crypted: 134.5 MB/s 127.9 MB/s
5 disks in raid-6 245.9 MB/s 221.9 MB/s crypted: 184.1 MB/s 189.1 MB/s
6 disks in raid-6 324.2 MB/s 315.8 MB/s crypted: 253.4 MB/s 236.3 MB/s
7 disks in raid-6 354.5 MB/s 350.6 MB/s crypted: 297.7 MB/s 293.3 MB/s
8 disks in raid-6 407.4 MB/s 419.8 MB/s crypted: 352.6 MB/s 351.3 MB/s
4 disks in raid-10 166.0 MB/s 159.4 MB/s
6 disks in raid-10 254.1 MB/s 257.4 MB/s
8 disks in raid-10 347.9 MB/s 316.2 MB/s
Some careful conclusions:
(1) when comparing RAID-5 and RAID-6:
RAID-5 and RAID-6 are comparable when you look at the same number of data disks (e.g. 2+1 in RAID-5 compares to 2+2 in RAID-6)
(2) when comparing the number of data disks:
increasing from 2 to 3, and again from 3 to 4 data disks, gives you a dramatic improvement in write speed (i.e. more significant than, say, WD red versus WD green?)
(3) comparing CPU's:
(a) the Atom is overall slower and runs out of steam beyond roughly 4 data disks in RAID-5 and RAID-6
(b) the G550 and i7 are much faster, both run out of steam beyond roughly 6 data disks in RAID-5 and RAID-6
(c) the i7 has a major advantage with encryption, due to its instruction set with AES support
(4) when considering RAID-10
The G550 and i7 CPU's are more than capable to perform the XOR operations at high speeds. In the numbers you can see that RAID-10 only gives a slight advantage when running on the Atom CPU. Also, note that only write speeds where compared. The striping advantage of RAID-10 is only effective when reading.
I have also looked at power consumption during these various tests. It looks to me that the G550 and i7 have a very much comparable power usage footprint in real life. Eventhough in theory the TDP of the G550 is higher, that doesn't mean that the real-life power usage (with RAID, in a NAS) is so much different between the i7 and G550. Of course if you look at power usage per unit of work, instead as of per unit of time, then the i7 wins because it's faster. At idle they're both comparable as well.
The numbers for a QNAP TS-870 with i3 CPU (the 870 Pro) would be interesting to add to this list. If anyone feels like wasting a full weekend doing such tests, please post your results here.