SSD DD write speed lower than HDD

Question

I've come across a strange phenomena with a new dedicated server that's been provisioned.

The server has 2x1TB HDDs in software RAID 1 and a separate 120GB SSD that is intended to be used as a FlashCache drive. Both the spinning disks and the SSD are SATA disks. Interestingly, it seems that write speeds to the SSD are approximately half those on the HDDs.

On the SSD (Toshiba Q300):

dd if=/dev/zero of=test bs=64k count=16k conv=fdatasync
16384+0 records in
16384+0 records out
1073741824 bytes (1.1 GB) copied, 13.9287 s, 77.1 MB/s

In contrast, same process on the HDDs:

dd if=/dev/zero of=test bs=64k count=16k conv=fdatasync
16384+0 records in
16384+0 records out
1073741824 bytes (1.1 GB) copied, 6.39255 s, 168 MB/s

These tests were taken before any FlashCache setup, on a new CentOS install.

Any ideas? I've been scratching my head over this. I've got a near-identical setup that has no such issues.

Do you have another SSD that you could replace it with to see if it's due to the disc or due to the setup? — Richard Boonen, Apr 29 '16 at 15:48
This should not be the case. Specially due to the overhead from RAID 1, the HDD would definitely run slower UNDER NORMAL CIRCUMSTANCES than the SSD. Try to run the test again and check your SATA ports. Maybe you accidentally got these figures interchanged. — jarvis, Apr 29 '16 at 16:15
Candidates to vote down. Too little info. What is the raid 1 cache setup? And the HDD connector? SATA? SAS? What is the SSD connected to? SATA? — mootmoot, Apr 29 '16 at 16:32
HDDs are running in mdadm RAID 1. No cache currently set up as said in the question, these are independent tests. Both SATA, SSD is a Toshiba Q300, I don't have access to the machine as it is a rented dedicated server — rich1334, Apr 29 '16 at 16:54
@HopelessN00b really unnecessarily vulgar language. I didn't have a choice in drive as I have already said, it's a leased dedicated server. I'll get in contact with the host. — rich1334, Apr 29 '16 at 17:00
Do you have a business reason to care about MB/s? The advantage of an SSD over an HDD is random IOPS, not sequential read/write speeds. And that's good, because no matter what you're doing with your server, you almost certainly care more about IOPS than MB/s. The fastest SAS drives perform at about 200 IOPS. Your cheap SSD is probably 300 times as fast as a best-in-class magnetic disk. That's what matters. — Skyhawk, May 01 '16 at 22:24

score 5 · Answer 1 · answered Apr 29 '16 at 17:19

The model of SSD in your server has well-known and widely-reported performance issues with writing large data sets. It's cheap junk that even manages to cost more than similar products without these issues. Replace it. (Or, get your host to replace it, in this case.)

SSD DD write speed lower than HDD

1 Answers1