io - Acceptable I/O speeds for 6 x 250GB SSDs in RAID 10

I'm running CentOS 7 (XFS filesystem) on a dell server with a PERC H700 raid controller. Inside this server I have 6 x Samsung 850 Evo 250GB SSDs (yes they are consumer drives however, this is a home server. In any case, I performed a DD test and am getting speeds of around 550MB/s which would be the approximate write speed of a single SSD yet these drives are in RAID 10.... where one would expect more.

Output of a write test:

[root@localhost] sync; dd if=/dev/zero of=tempfile bs=1M count=1024; sync
1024+0 records in
1024+0 records out

1073741824 bytes (1.1 GB) copied, 1.95942 s, 548 MB/s

Output of a read test:

[root@localhost]# dd if=tempfile of=/dev/null bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 0.171463 s, 6.3 GB/s

Would anyone be able to shed some light on this situation as to whether this is an acceptable write speed? I'm rather puzzled as to what to do here.
Appreciate your help :)

Answer

I could close this as a duplicate because there are a lot of factors that impact storage performance in Linux.

I think people have the wrong idea when they attempt to benchmark SSD performance. You should use SSDs for better random I/O performance. You're testing big-block sequential performance, which doesn't match any sort of use case except for, um, copying large files.

• Throughput: Maximum bandwidth (likely sequential) of the array.



• IOPS: How many I/O operations per second the array is capable of.


• Latency: How quickly the storage subsystem can service your I/O requests.

The last two are what matter in most cases. Add to this the fact that you're using a RAID controller, there is an element of caching at play. Also, XFS and Linux cache I/O, so you need to know what you're testing.

I'd suggest using a purpose-built tool like fio, iozone or even bonnie++ to run a proper set of benchmarks.

Also see: HP P410 RAID + Samsung 830 SSDs + Debian 6.0 - What performance to expect?