WD Black SN770 or Crucial P5 Plus 1TB or something else?

[Mista G]

Currently gathering hardware for a new build and next up is a gen 4 SSD. I have a couple of choices between the WD Black SN770 1TB and Crucial P5 Plus 1TB. Which one do I buy or perhaps neither and recommend me another?

[mariushm]

I would say WD SN770 is a bit better than Crucial P5 Plus

 

Crucial P5 Plus has slightly higher read speeds ( 6 GB/s vs 5..5.5 GB-ish for SN770) but that's only achieved when reading big chunks of data, not smaller chunks that would be normal when loading games, or doing things with computer on average. 

See the picture below from Tom's Hardware Review - when application/game reads in chunks of 512 KB or less, the SN770 (pink and red lines) has the edge, it can read at faster speeds than Crucial - the Crucial drive (in orange) starts to achieve higher than 4 GB/s reads only when reading chunks bigger than 512 KB which is basically only when you copy files or maybe when doing some video rendering, stuff like that.

 

 

 

When writing there's also some difference. Writing to TLC memory is slow, it's not really possible to achieve 4-5 GB/s write speeds writing directly in TLC memory. 

Drives have a SLC cache portion - basically a portion of the unused memory on the drive is "switched" into a much faster mode and used as a write cache. For example, 100 GB of free TLC memory can be converted into 25-40 GB of SLC write cache. 

So, as long as there is free space in that region of write cache memory, you get fast write speeds. When that area gets filled, the write speeds decrease, and in background the drive starts moving the data from that write cache to permanent TLC memory and then the speeds go fast again.

 

Crucial P5 Plus 1 TB seems to have up to around 100 GB of write cache, and writes in it at around 5.1 GB/s but once that's filled, it starts writing directly to TLC memory at around 1.2 .. 1.5 GB/s - in the picture below it's in TEAL (light blue), y axis is the speed in MB/s , on the x axis is time in seconds

 

The SN 770 can use much more free memory as a write cache, the review says up to 377 GB with the drive nearly empty.  They filled nearly 400 GB at almost 5 GB/s but then the write speeds dropped to around 600 MB/s when switching to writing directly to TLC memory. 

When the drive is idle, it starts moving that cached data to TLC memory at a rate of around 30 GB per minute, and the review says that as long as the write cache is only partially full, you get around 1.5 GB/s write speeds. 

 

So with the SN770 you could install a couple of 70-100 GB games at 4-5 GB/s and the write cache would still not be full, so you will rarely notice the write speeds dropping to 600 MB/s levels. 

 

Of course, when the drive gets full, this amount of write cache will decrease - if you have 600 GB out of 1 TB full, the write cache will probably be only around 30-50 GB

 

[Mista G]

@mariushm Thank you for such a detailed response, it appears you've made my choice!