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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.

closed as off-topic by HopelessN00b, Ward, Jim B, fuero, MadHatter May 3 '16 at 12:31

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  • 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
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    "Why is my pickup truck faster than my convertible?" – HopelessN00b Apr 29 '16 at 16:11
  • 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
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    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
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    And type of SSD, brand of SSD. etc – mootmoot Apr 29 '16 at 16:43
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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.)

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