Why RAM disk on MBP 16" 2019 is slow?

[hajime]

Hi, on my MBP (i9-2.4, 32GB, 2TB), I recreated a ram disk of size 1280MB using:

diskutil erasevolume HFS+ 'RAM Disk' `hdiutil attach -nomount ram: //2621440`

where Size of RAM disk desired * 2048 = amount to specify. 2621440 = 1280*2048 = 2621440.

When I ran Disk Speed Test, the max rad and write speeds of the RAM disk were around 5000 MB/s.

However, in the following videos, the guy's RAM disk reached the max. How come? The MBP 16" is probably faster than his.

For reference, when running Disk Speed Test, the write speed was about 28xx MB/s while the read speed was about 27xx MB/s. So, the SSD speed of mine is faster than his but his RAM disk is much faster?!

 

[junkw]

what happens if you create a 10GB ram disk

 

[hajime]

what happens if you create a 10GB ram disk

Write: 5000-5600 MB/s
Read: around 4700 MB/s

 

[joelhinch]

Im only getting 1600 write and 5600 read. The machine feels sluggish so i'm not sure what is going on.
64GB RAM model.

 

[jinnyman]

I'm not familiar with ram disk. Never tried. So I'm just guessing.
Perhaps iGPU graphic sharing system ram is something to do with?

no idea

 

[joelhinch]

I'm not familiar with ram disk. Never tried. So I'm just guessing.
Perhaps iGPU graphic sharing system ram is something to do with?

no idea

since you have a 16" would you mind creating a ramdisk then testing the write speed with an app like blackmagic speed test?

can create a 10GB ramdisk with a terminal command

diskutil partitionDisk $(hdiutil attach -nomount ram://21845333) 1 GPTFormat APFS 'ramdisk' '100%'

just kill the process from activity monitor after the testing to clear it up

 

[robvas]

OS difference?

 

[bennyprofane]

It fluctuates greatly but the max I've gotten is around

5600 write / 7200 read

 

[leman]

I don’t think there is anything out of ordinary here. Overhead of the file system etc.... the numbers seem reasonable, you are getting around 15-20% bandwidth of what your RAM can theoretically handle. The fact that he is getting slightly higher numbers is of no importance IMO.

The most interesting part for me here is that RAMdisks are almost pointless nowadays given the SSD speeds.