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I purchased a Dell EMC R740 with 8 Intel S3520 SATA 6 Gbps SSD drives. The problem I have is that no matter what RAID configuration I select I seem to be limited to roughly 5 Gbps. I would think that 8 drives in a raid 5 configuration would operate at 6 Gbps x 8 = 48 Gbps. I can understand a little loss for parity bit calculations but I should be seeing better performance. The controller card is rated for 12 Gbps so I would expect to see that speed at a minimum. We have tried various RAID modes and settings on the controller with no luck.

We also purchased 8 Seagate Nytro 3730 SAS 12 Gbps SSD cards and put them in the server with the exact same results.

We have spend hours on the phone with Dell and they said that the SSDs are rated at 6 Gbps and that is all they will do no matter the RAID configuration. When we told Dell we purchased new drives rated at 12 Gbps and they were not working either they said there is code in the controller to keep it from performing with non-Dell drives.

Am I correct on my raid/speed thoughts? Is there a different controller that will perform better? Any other ideas?

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First of all, SATA 6Gbps is maximal theoretical speed of interface and not throughput of SSD!

According to product specification, one S3520 can show maximum 450MB/s (3.6 Gbit/s) for reads and 380MB/s (3 Gbit/s) for writes.

Second thing, how is RAID5 configured (stripe size, write and read policies)?

And the third - what patterns did you use to test performance? If you has measured 4k random write – 5 Gbit/sec is 10 times more as theoretical value for such configuration.

In conclusion – I used to achieve approx. 3.5 Gbyte/s (sequential read, 64k) with PERC 730 and 8 SSD SATA in RAID5 in systems configured for customers

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I have a similar system, but it uses Intel DC S3100 SATA drives.

[    0.000000] DMI: Dell Inc. PowerEdge R740/00WGD1, BIOS 1.3.7 02/08/2018
[    3.780967] scsi 1:2:0:0: Direct-Access     DELL     PERC H730P Adp   4.27 PQ: 0 ANSI: 5

Measure the performance in RAID 1, using 2 drives (a simple mirror):

# dd if=/dev/zero of=/tmp/test.img bs=1M count=20000
20000+0 records in
20000+0 records out
20971520000 bytes (21 GB) copied, 51.538 s, 407 MB/s

And RAID 10 using 6 drives (3 drive stripe, with mirroring):

# dd if=/dev/zero of=/tmp/test.img bs=1M count=20000
20000+0 records in
20000+0 records out
20971520000 bytes (21 GB) copied, 17.2245 s, 1.2 GB/s

You can see that the drives performed about 3x better than a single drive, as would be expected from SSD's as they scale close to linearly when striped.

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