Speed of blade SSD on 5k Retina 27in iMac?

[Poncho]

I bought a 5k Retina 27in iMac when it came out and chose a 2TB Fusion drive model to keep the cost down. A couple of months ago the spinning HD failed inside it. Anyway, I had this replaced with a 2TB Samsung SSD QVO. So now have a fusion drive comprising this and the original 125GB Apple blade SSD. When I run Blackmagic Disk Speed Test I get reads around 400 and writes around 500 (see my attached pic). Yet I have seen vids on YouTube of standard non-modified Fusion 5k Retina 27in iMacs with read and write speeds in the hundreds and thousands. Like here:

Does anybody have any idea what's going on here? I hope that the 125GB Apple blade SSD is actually running at these very high speeds but that the Blackmagic Disk Speed Test is writing to the 2TB Samsung SSD QVO as there is no room on the 125GB Apple blade SSD. But there is no way of testing the 125GB Apple blade SSD on its own.

 

[Fishrrman]

Is the internal blade SSD "fused with" the SATA SSD?
Do you have one drive icon on your desktop?
Or... do you have TWO?
(if the drives are fused, there would be one. If they are not, there should be two)

Open disk utiity.
Go to the "view" menu and choose "show all devices".
Take a screenshot of what disk utility is showing you, and post it here.

 

[Poncho]

Is the internal blade SSD "fused with" the SATA SSD?
Do you have one drive icon on your desktop?
Or... do you have TWO?
(if the drives are fused, there would be one. If they are not, there should be two)

Open disk utiity.
Go to the "view" menu and choose "show all devices".
Take a screenshot of what disk utility is showing you, and post it here.

Here we go. It's fused...

 

[Fishrrman]

I would do this (and yes, it involves some work):
1. Back up your data (this is a MUST)
2. DE-fuse the drives... split them apart. You now have the fast internal blade and the slower SATA SSD.
3. Install your OS onto the blade. It should have the OS, your apps, and a basic account (not a lot of stuff in it).
4. Keep your large libraries of photos, music, movies, etc. on the SSD.

This should speed up the Mac considerably.
Of course, you have to manage two drives instead of "one".
Nothing to it.

 

[Brian33]

If the Fusion drive were set up properly, I don't think splitting it up would be likely to increase the storage performance for the BlackMagic disk speed test, at least not with files 4 GB or smaller. A 4GB file will normally be written entirely to the (faster) internal blade. This is because Fusion keeps a 4GB buffer free on the faster portion. Then using idle cycles moves less-frequently-used blocks from the faster component to the slower one, to free up 4 GB again. (My experience is with Fusion drives in the CoreStorage/HFS+/High Sierra setup, but I don't think that behavior has changed with APFS in Mojave & Catalina.)

I wonder if, for some reason, the Fusion drive is set up backwards, designating the SATA SSD as the "faster" component and the blade as the "slower". It seems to me that would account for the disk test results.

What year is your iMac? What macOS are you running? I don't have a Fusion drive in APFS right now, but I think that the Terminal.app commands "diskutil apfs list" and "diskutil apfs info <UUID>" will probably give information about the Fusion setup.

 

[Brian33]

Also, his screen shot in Disk Utility does not have "View-->Show All Devices" selected. The volume called "Fusion Drive" could be entirely on the new SSD, with no volume at all on the internal blade. (I.e., there no actual Fusion Drive set up, just a disk label.) That is probably the most likely explanation for his performance results.

OP -- go to Disk Utility's "View" menu and check off "Show all devices". It should show two top-level physical storage devices, not a single Fusion Drive.

 

[Poncho]

OP -- go to Disk Utility's "View" menu and check off "Show all devices". It should show two top-level physical storage devices, not a single Fusion Drive.

Running Sierra on a 2017 inch 5k iMac Retina so Disk Utility doesn't have a 'View' menu (see attached shot). Intrigued with your idea that the Fusion drive could be set up backwards. Can I see this in Terminal?

 

[rbart]

It should be faster.
I have better performance with a FD of 24gb blade +1tb 860evo (iMac 27 late15)

 

[Poncho]

Actually, Terminal has the Apple SSD listed first

 

[Brian33]

Ok, Sierra, so you're using CoreStorage. Hmmm. The FD looks OK, but I would not assume that the order of the devices in the output is related to the how the FD is set up.

What does the output of 'diskutil cs info <uuid>' where <uuid> is the UUID for the logical volume 0AF8A6..... ?
I think that command gives more details about the logical volume.

I'm wondering how the FD got set up. Here's Apple's instructions:
https://support.apple.com/en-us/HT207584

In the command diskutil cs create Macintosh\ HD identifier1 identifier2
I had thought that it was important that 'identifier1' be the fast device (i.e., that the blade device be listed first), but I see that Apple doesn't actually say that on that page, so maybe it doesn't matter. But maybe it does...

 

[Brian33]

It is possible to watch a FD in action, using the 'iostat' command in Terminal:

1) Use 'diskutil list' to find out for sure which device is disk0 and which is disk1 (you might want to eject your Samsung T7 to avoid confusion). Note that reboots can cause the device numbers to change, so check that you know which is which.

2) In Terminal, run iostat -d -w 1

3) Start writing/copying a big file -- 5GB or more. I guess BlackMagic at the 5 GB setting could work.

4) Watch the output of the iostat command: you should see lots of tps and MB/s show up mostly under either disk0 or disk1. That's the "fast" part of the FD.

5) After about 4GB has been written, you'll see the tps and MB/s numbers under the other disk go way up. That's because the buffer on the "fast" device has been filled, and data is now being written to the "slower" portion of the FD.

6) When you're done hit Ctl-C in the terminal window to stop iostat.

(All the above assumes you have more than 128 GB of data on the FD. That is, that the "fast" portion is filled except for the 4 GB buffer.)

 

[Poncho]

It is possible to watch a FD in action, using the 'iostat' command in Terminal:

1) Use 'diskutil list' to find out for sure which device is disk0 and which is disk1 (you might want to eject your Samsung T7 to avoid confusion). Note that reboots can cause the device numbers to change, so check that you know which is which.

2) In Terminal, run iostat -d -w 1

3) Start writing/copying a big file -- 5GB or more. I guess BlackMagic at the 5 GB setting could work.

4) Watch the output of the iostat command: you should see lots of tps and MB/s show up mostly under either disk0 or disk1. That's the "fast" part of the FD.

5) After about 4GB has been written, you'll see the tps and MB/s numbers under the other disk go way up. That's because the buffer on the "fast" device has been filled, and data is now being written to the "slower" portion of the FD.

6) When you're done hit Ctl-C in the terminal window to stop iostat.

(All the above assumes you have more than 128 GB of data on the FD. That is, that the "fast" portion is filled except for the 4 GB buffer.)

Super stuff!!! I'll do all this and report back! Thanks.