Data Transfer rate correlation

[yurayurayur]

Hi,
I hope this belongs to this forum but I couldn't find anywhere more suitable to post this.
I have a question that been bugging me for the logest time and everyday I learn more and more because I work in the IT industry but never manage to answer this question.
Can someone explain me the connection from the hard drive to the Southbridge/CPU. what I mean is this:
lets say SAS drive runs at 15K RPM, the data transfer rate is SAS III which is 6Gbp/s and if we look at a good DELL RAID controller, such as the H700, which is also 6Gbp/s, the bus interface is PCI-e 2.0 x8 which means 4Gbp/s of transfer from the controller to the chipset and to the RAM/CPU.
can someone elabore for me how the data transfers and connects between all of these speeds? because they are all different.
if someone got an illustration, it would be highly appreciated.

Thank you for your time.

 

[Yami]

Hi,
I hope this belongs to this forum but I couldn't find anywhere more suitable to post this.
I have a question that been bugging me for the logest time and everyday I learn more and more because I work in the IT industry but never manage to answer this question.
Can someone explain me the connection from the hard drive to the Southbridge/CPU. what I mean is this:
lets say SAS drive runs at 15K RPM, the data transfer rate is SAS III which is 6Gbp/s and if we look at a good DELL RAID controller, such as the H700, which is also 6Gbp/s, the bus interface is PCI-e 2.0 x8 which means 4Gbp/s of transfer from the controller to the chipset and to the RAM/CPU.
can someone elabore for me how the data transfers and connects between all of these speeds? because they are all different.
if someone got an illustration, it would be highly appreciated.

Thank you for your time.

I just want to point out that PCI-E 2.0 x8 is 4GB/s (Gigabytes per second) rather than 4Gb/s (Gigabits per second).

Otherwise, I'm not really sure what you're asking? Are you asking how the data flows and what the real-world transfer speed would be of such a system?

 

[yurayurayur]

Hi,
thanks for the reply. yes! you are right, I meant 4GBp/s.
my question is how the data flows from the hard drive to the RAM at any point at what speed.
lets say I ask you, how it is all connected : 15K RPM, 6Gbp/s, 4GBp/s.
perhaps in time matter. also, there is the cache on the controller, 512MB or 1GB. how is that connected to the equation here?
I know this question is a little bit tricky but I'm sure there's an answer to it. I just couldn't search it on google. dont even know how to search for it on google

Thank you in advance.

 

[Yami]

Well, you seem to have most of the idea.

Step 1) 15K RPM drive. This is probably going to be the slowest device in the chain; even though it's on a 6Gb/s bus, the read speed of the platters in the drive will be slower than that. A top-end 15K SAS drive will get about 130MB/s read speed, if I remember right.
Step 2) SAS cable. This goes at 6GB/s (or 12Gb/s) to...
Step 3) Dell RAID card, goes at 6Gb/s to...
Step 4) PCI-E 2.0 x8 bus, goes at 32Gb/s to...

Now, the next step depends on the motherboard's chipset. Typically it will get sent to the southbridge, as shown here (https://upload.wikimedia.org/wikipedia/commons/e/ed/Schema_chipsatz.png), and then on to the northbridge (often integrated onto the CPU nowadays), where it gets sent wherever needed.
I'll use modern Intel chipsets as an example, where the southbridge and northbridge (now integrated) are connected via a BUS called DMI, or Direct Media Interface. This has a transfer speed of 32Gb/s.

Step 5) The southbridge, goes at 32Gb/s via DMI to...
Step 6) The northbridge (now CPU-integrated), goes via the QPI at upward of 200Gb/s to...

Next up is the bus between the CPU and memory. In Intel's case, the QPI or QuickPath Interconnect is used. The transfer speed is variable - the Wikipedia explains how it's calculated but it's generally around 200Gb/s.

Step 7) The memory! Writes at speeds of 12.8GB/, or around 100Gb/s in the case of DDR3-1600.


I hope this is helps and is what you were after.

 

[yurayurayur]

thanks for the explanation. one last question about that.
according to this, if I try to open a file and load it to the RAM, a picture for example that weights 5MB, it would take me 5MB divided by 130MB milliseconds to seek it within the drive, then 5MB divided by 6Gbp/s milliseconds to transfer it within the controller and then 5MB divided by 32Gbp/s milliseconds to transfer it to the southbridge? can you confirm that this is how it works?

on an opposite example, lets say the file is 520MB, then it would take me 4 seconds to read it from the drive and then almost a second to transfer it within the controller(6Gbp/s is 720MBp/s approximately so almost one second) and then 1/8 of a second to transfer it from the controller through the BUS so in total it would me around 5 second to load a file from the SAS drive to the RAM according to this. is this how its going to work?

Thank you in advance.

 

[Yami]

thanks for the explanation. one last question about that.
according to this, if I try to open a file and load it to the RAM, a picture for example that weights 5MB, it would take me 5MB divided by 130MB milliseconds to seek it within the drive, then 5MB divided by 6Gbp/s milliseconds to transfer it within the controller and then 5MB divided by 32Gbp/s milliseconds to transfer it to the southbridge? can you confirm that this is how it works?

on an opposite example, lets say the file is 520MB, then it would take me 4 seconds to read it from the drive and then almost a second to transfer it within the controller(6Gbp/s is 720MBp/s approximately so almost one second) and then 1/8 of a second to transfer it from the controller through the BUS so in total it would me around 5 second to load a file from the SAS drive to the RAM according to this. is this how its going to work?

Thank you in advance.

Pretty much. There are ways to make it much more complicated but that should be plenty for your needs.