ahci sm951 drop half speed on mac pro 5.1

[parisinvest]

hello you all, i've fixed the ancien start problem on my mac pro 5.1, but when i checked the components, i've found the speed of my ahci sm951 has droped to half, it was +1400mb/s read and write, but now only have +700mb/s, did not change anything in the macpro, it's so wired, anyone knows this issue and a way to fix it please? many thanks, daniel

 

[bookemdano]

What slot do you have it in? Try it in slot 3 or 4.

 

[bsbeamer]

Only time I’ve experienced anything like this was after a software update around 10.13.2 or 10.13.3. Read/write speeds dropped in more than half. Was Samsung SSD (SATA) via PCIe.

Cloned boot drive to another SSD formatted HFS+ (not APFS) and replaced. Immediately fixed all issues. Same drive running 10.13.6 now with HFS+.

If you’re on APFS, may be at least worth looking at.

 

[h9826790]

hello you all, i've fixed the ancien start problem on my mac pro 5.1, but when i checked the components, i've found the speed of my ahci sm951 has droped to half, it was +1400mb/s read and write, but now only have +700mb/s, did not change anything in the macpro, it's so wired, anyone knows this issue and a way to fix it please? many thanks, daniel

you may install the PCIe SSD in slot 3/4, or update your cMP firmware to 138.0.0.0.0

 

[parisinvest]

What slot do you have it in? Try it in slot 3 or 4.

moved to slot 4, problem solved, thank you, but still cant understand why it worked on slot 2 16x before, and now have to work on slot 3/4?
[doublepost=1536843639][/doublepost]

you may install the PCIe SSD in slot 3/4, or update your cMP firmware to 138.0.0.0.0

I've moved it to slot 4, problem solved, thank you so much, but still don't know why it doesn't work on 16x anymore? btw, if I upgrade the fw, can I still keep my sierra or high sierra ?
[doublepost=1536844396][/doublepost]

Only time I’ve experienced anything like this was after a software update around 10.13.2 or 10.13.3. Read/write speeds dropped in more than half. Was Samsung SSD (SATA) via PCIe.

Cloned boot drive to another SSD formatted HFS+ (not APFS) and replaced. Immediately fixed all issues. Same drive running 10.13.6 now with HFS+.

If you’re on APFS, may be at least worth looking at.

thank you for you help, I've simplely changed the slot and the problem solved, why? I don't know why 16x is slower than 4x...

 

[h9826790]

moved to slot 4, problem solved, thank you, but still cant understand why it worked on slot 2 16x before, and now have to work on slot 3/4?
[doublepost=1536843639][/doublepost]

I've moved it to slot 4, problem solved, thank you so much, but still don't know why it doesn't work on 16x anymore?

The SSD is PCIe 3.0 x4, put it in a x16 slot won't help anything.

However, due to the cMP firmware issue, most PCIe 3.0 device cannot properly negotiate at PCIe 2.0 speed in slot 1 and 2. Resulting further fall back to PCIe 1.0 link speed. Which means your SSD fall back to PCIe 1.0 x4, theoretical max speed is 1000MB/s, real world max at ~750MB/s.

This issue, however, can be fixed by put a PCIe switch inside the connection. e.g. If you use the Highpoint 7101A adaptor, due to the onboard PCIe switch, the SSD can actually negotiate at correctly link speed.

To avoid this issue, you can install the SSD in slot 3 or 4. Due to slot 3 and 4 share use the same PCIe 2.0 x4 lane. Therefore, there is a build in PCIe switch, which helps the SSD to negotiate at the correct link speed.

To permanently fix the issue, you can upgrade the cMP firmware to 138.0.0.0.0. At this moment, this version may be considered as a "beta" firmware due to only available at the 10.14 beta installer. But this should become formal soon.

 

[amarmot]

However, due to the cMP firmware issue, most PCIe 3.0 device cannot properly negotiate at PCIe 2.0 speed in slot 1 and 2. Resulting further fall back to PCIe 1.0 link speed. Which means your SSD fall back to PCIe 1.0 x4, theoretical max speed is 1000MB/s, real world max at ~750MB/s.

Does this also impact x8 devices such as OWC Accelsior 4M2?

 

[h9826790]

Does this also impact x8 devices such as OWC Accelsior 4M2?

This only applicable to some older firmware. The latest 144.0.0.0.0 BootROM already fixed this issue.