Speed difference between two Apple SSDs

[alex87f]

Hello everybody,

I recently bought a very lightly used early 2015 13" MBP with the stock 128gb SSD, and then upgraded it with a used 256gb apple SSD (also an SSUBX - PCIE 3.0 drive).

I ran comparative black magic tests on the old and the new drive and am a little surprised with the results:
-Old (2015/08) SSUBX 128gb drive: 650mb/s write - 1300mb/s read
-"New" (2016/04) SSUBX 256gb drive: 1250mb/s write - 1400mb/s read

Both test were done on clean Mojave installs. I'm happy with the result but would like to understand why there's such a difference on the write speeds. Any ideas?

 

[Audit13]

Write speeds will go up as the size of the drive increases, just like a 512 GB drive will have faster write speeds than a 256 GB equivalent model.

The speed increase is due to the way SSDs write data.

 

[Weaselboy]

Totally normal like @Audit13 said. The memory chips are laid out in parallel and can be written to at the same time. So a smaller SSD might only have four chips where the larger has eight (made up example) allowing higher write speeds.

 

[robvas]

Hello everybody,

I recently bought a very lightly used early 2015 13" MBP with the stock 128gb SSD, and then upgraded it with a used 256gb apple SSD (also an SSUBX - PCIE 3.0 drive).

I ran comparative black magic tests on the old and the new drive and am a little surprised with the results:
-Old (2015/08) SSUBX 128gb drive: 650mb/s write - 1300mb/s read
-"New" (2016/04) SSUBX 256gb drive: 1250mb/s write - 1400mb/s read

Both test were done on clean Mojave installs. I'm happy with the result but would like to understand why there's such a difference on the write speeds. Any ideas?

While in your particular case you are seeing the increased speeds of the larger drives (which the other posters mentioned), if you took an SSD from a MacBook Air and installed it in your Pro, iit might only use 2 PCI lanes instead of 4 and you would also see a larger performance decrease.