M2 SSD slows down big time with large file writes?

[SPL Tech]

I am running a 500GB M2 SSD as my secondary drive. It does not have the OS on it. It's just an extended partition for saving files. Write speed normally benches at just under 1 GBPS. However, if I try to write a 250 GB file to it what I notice is after about 50 GB, it starts to slow down progressively. 800 mb/s, then 600, 500, 400, ect uniformly until it gets down to 50 mb/s. What's the deal? I though these things were supposed to be lightning fast.

First thought was it was overheating. The SSD does get quite hot after 10 min of constant writing of files. However, my mobo has heat spreaders for all of the M2 drives so I would think that would solve any heat issues.

 

[mackerel]

They are fast, but not when you write to half the drive in one go. Assuming it is a recent consumer grade drive, it'll have a write cache of some sort intended to deal with normal use. A large write will fill that and it'll fall back to the native baseline flash write speed. Cheaper flash in consumer drives isn't so fast once the cache is filled.

Heat could also be a factor. Actually monitor it to be sure rather than think just because you got some kinda heatsink on it that it will be fine.

 

[EarthDog]

However, my mobo has heat spreaders for all of the M2 drives so I would think that would solve any heat issues.

That is an assumption...

I'd take a look at drive temps...sounds like that kind of throttling if it constantly gets slower. Usually when you run out of cache it goes steady state without a decline.

 

[SPL Tech]

I tried copying a smaller file today--80 GB. I noticed that after about only 20 GB (which is literately only 20 seconds) it starts to slow down gradually. This was from a cold start where I booted the computer when it was at room temp and started copying the file as soon as I was in Windows. I checked the temp of the SSD and after the 80GB transfer was done the temp was 55C. So I am not sure what gives.

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They are fast, but not when you write to half the drive in one go. Assuming it is a recent consumer grade drive, it'll have a write cache of some sort intended to deal with normal use. A large write will fill that and it'll fall back to the native baseline flash write speed. Cheaper flash in consumer drives isn't so fast once the cache is filled.

Heat could also be a factor. Actually monitor it to be sure rather than think just because you got some kinda heatsink on it that it will be fine.

Any ideas on how much cache is in a modern SSD and what the raw speed is of the flash memory? Say on something like an mid level WD NVMe SSD. 960Evo or something of the sorts.

 

[mackerel]

Any ideas on how much cache is in a modern SSD and what the raw speed is of the flash memory? Say on something like an mid level WD NVMe SSD. 960Evo or something of the sorts.

You will have to look it up for the exact model and capacity in question. The cache size may not be constant either. Drives may dynamically allocate more of the capacity to cache when it is relatively empty, compared to nearly full.

Since you mention the 960 Evo, and I have one of those, I did look that one up. From: https://www.anandtech.com/show/10833/the-samsung-960-evo-1tb-review
1 TB model has 42GB cache, 1200MB/s sustained (after cache)
500GB model has 22GB cache, 600MB/s sustained
250GB model has 13GB cache, 300MB/s sustained

Note this is a relatively above average SSD. Newer drives using QLC can be much slower in writes once the cache is filled.

 

[EarthDog]

I checked the temp of the SSD and after the 80GB transfer was done the temp was 55C. So I am not sure what gives.

after? Like checking the cpu temp a couple seconds after the test and it drops a couple dozen degrees? M.2 is not a cpu, but make sure you capture peak temps when it's running.

Flash memory sizes vary by drive (some don't have any).

 

[CJustin]

I have observed the same performance drop on my M.2 drives as well.
It is because when you write a huge file to the drive it takes time to write all the data to the drive.
Most of the data needs to be cached to a sub-dynamic station and then it is written to the drive.

I suggest you break the file into multiple small parts and then write them to the drive.
That would be much faster than putting the whole file on the drive at one time.

 

[EarthDog]

I have observed the same performance drop on my M.2 drives as well.
It is because when you write a huge file to the drive it takes time to write all the data to the drive.
Most of the data needs to be cached to a sub-dynamic station and then it is written to the drive.

I suggest you break the file into multiple small parts and then write them to the drive.
That would be much faster than putting the whole file on the drive at one time.

????

No.