HDD write speed ?

[Thony]

When I copy files to my HDD shouldnt that be limited to the HDD's write speed ?
I thought it should but copying files from my SSD to HDD I reach SSD speed (550MB/s). What is the write speed for then ?

[AresKrieger]

You likely have RAM caching being displayed as written data if it is exceeding the HDD's write speed, as the write speed is the max speed data can be written to the drive.

[WkdPaul]

Thread moved to Storage section

[Gonio]

The SSD is doing the writing to your HDD. So the speed is depending on your SSD.

[Thony]

2 minutes ago, Gonio said:

The SSD is doing the writing to your HDD. So the speed is depending on your SSD.

So write speed is only limited to HDD's write speed if the HDD does the writing ? I thought HDD is doing the writing every time new data is written on it. Interesting.

[Wolther]

7 minutes ago, Gonio said:

The SSD is doing the writing to your HDD. So the speed is depending on your SSD.

That's not how it woks. There's a reason there's a write head. That write head writes to the platters and its limited to the hdd it self. The ssd can send data but it's dependent on the hdd's speed 

[Thony]

2 minutes ago, Wolther said:

That's not how it woks. There's a reason there's a write head. That write head writes to the platters and its limited to the hdd it self. The ssd can send data but it's dependent on the hdd's speed 

Then why was I writing a 2GB file at speed of 550MB/s for 4 seconds instead of 12 ?

[Gonio]

26 minutes ago, Wolther said:

That's not how it woks. There's a reason there's a write head. That write head writes to the platters and its limited to the hdd it self. The ssd can send data but it's dependent on the hdd's speed 

It kinda is. the HDD is the receiving side, the SSD writes the file towards the HDD while the HDD accepts the delivery trough the SATA connection. Thats how I understand it atleast but correct me if I am wrong.

 

You can try it easy: take a single video file from your SSD and copy it to a HDD, voila OP's situation. Try the reverse and the speed will be lower.

[Thony]

21 minutes ago, Gonio said:

It kinda is. the HDD is the receiving side, the SSD writes the file towards the HDD while the HDD accepts the delivery trough the SATA connection. Thats how I understand it atleast but correct me if I am wrong.

 

You can try it easy: take a single video file from your SSD and copy it to a HDD, voila OP's situation. Try the reverse and the speed will be lower.

You are right about trying the reverse method. 

[pyrojoe34]

AresKrieger is correct, this is because some of the data is being cached before going to the HDD platter. Yes, you are still limited by HDD write speeds but the SSD can read at full speed, store the data in the cache, then the HDD can write the data at it's own pace. You will often see the first few megs of a transfer go much faster then the disk can possibly write, then once the cache is full it will slow to the speed the disk can write at.

 

Look at this image of me transferring a file from my SSD to a (very) slow 5400rpm HDD:

The red portion is where the data is being written to the cache so it can write very fast (looks like 350+ Mbps, much faster then the HDD could write to the platter). Where the blue starts is where the cache runs out of space and must wait until what is in it is written to the platter before it can accept more data. At that point the transfer is limited to the write speed of the platter and will remain at that speed until the transfer is done and the cache is freed up.

 

You can test this by playing around with file size. If the disk has a 64MB cache you can likely transfer a 30MB file at like 350-500MBps, however, if you transfer a 1GB file you will be limited to whatever your disk write speed is (maybe 150MBps or much less depending on the drive).

 

The reason it doesn't work in the other direction is because the data is not preloaded to the cache (it has no idea what data you want to access before you access it so it won't be loaded) so it will be limited to the read speeds of the drive from the start without the burst phase you see when writing.