SSD gets very slow when copying files

[RoDeLoPo]

Hi everyone. I am experiencing some really strange behavior with my 840 Evo. Sometimes, when I copy large amounts of files, the transfer speed will drop significantly. It usually start around ~300Mbps, then drops to around ~10Mbps, sometimes even reaching the whopping speed of 500Kbps. Can anyone explain this behavior? Thanks!

 

[theunliked]

That is normal, because of the cache. The ssd cache is much faster than the ssd itself. So when it starts, it fills up the cache. Then, the data spills into the ssd itself, slowing the copy down. The cache helps when you copy files smaller than the cache size, making the transfer almost instant. It can then slowly copy it over to the ssd while you do other stuff.

 

[nukemaster]

Smaller files are always harder on copy operations than large ones as well.

500kilobytes/s seems slow, but remember SSDs are FAST, but tend to write slower than they read.

Always try to keep some spare space for the drive to do housekeeping and please ensure you are in AHCI mode.

 

[Solandri]

An 840 EVO scores 35+ MB/s reads and 75+ MB/s writes with 4k files, so no it shouldn't be slowing down even to 10 MB/s on small files.
http://hexus.net/tech/reviews/storage/58229-samsung-ssd-840-evo-120gb/?page=5

How full is the drive? SSDs are not like HDDs where you can overwrite old data with new data. SSDs need to first erase the old data before they can write new data. That erase operation is a lot slower than a write operation. If the SSD has lots of empty space, then it can erase that free space during idle time so it's all prepped for fast writes.

But if the SSD is almost full and you've just deleted a bunch of files to make space to write new ones, then it needs to erase that space first before it can write the new files. That will go really slow. 0.5 MB/s sounds about right, though it depends on the SSD.

Try to keep at least 15% of the SSD free to avoid these slow writes. Make sure TRIM is enabled (not as important with newer drives, but it still helps). If you're using them in unusual configurations like RAID which don't support TRIM, keeping enough free space is even more important.
http://lifehacker.com/5640971/check-if-trim-is-enabled-for-your-solid-state-drive-in-windows-7

 

[nukemaster]

I will use an example here.

My good old M4 128 gigabyte SSD shows 4k write speeds of 54 megabytes/sec yet copying something with very small files(from a ramdrive to remove bottlenecks as much as I can). It starts fast, but falls down to 4.77 megabytes/sec. This was only 150 megabytes of files(15039 files in total). If I copy over my Windows 10 iso, it will get done faster because the drive will transfer data faster.

Smaller files are just slower. I would run a benchmark or 2 to ensure you are in the right ballpark too just in case.