11 replies to this topic

[vijay.gupta]

I observe a behavior when I copy any big file from pen drive to computer or vice versa or from one location in Windows to other location. When I do this, during the initial few seconds, the transfer rate is very high but as the time passes by, the rate decreases. why is this happening?

[terry1966]

for a simple question that needs a very complicated answer and one i'm not sure i can fully answer.. but this is how i think of it.. when it first starts, it's reading the data from one medium before copying it to the other, now as much of this data is read and put into memory as possible, which is very fast(so the data transfer speed is high) when the memory is full, it has to start writing that data to where you want it(writing data is always slower than reading data) so now the transfer slows down(and usually remains pretty steady) to what ever the maximum write speed is for that medium(disc/flash drive,etc), in the time it takes to write the data from memory the pc can read the next part of the original file and fill the memory back up again ready to repeat the process until the file transfer is completed.

[vijay.gupta]

3 queries in your reply: 1) by memory, are you referring to clipboard in the above post? 2) going by the explanation, the speed should fluctuate but it does not happen? 3) can you please explain "reading data is always faster than writing data". What is the rationale behind this?

[terry1966]

the memory i'm talking about is the actual physical memory inside a pc, reading is faster because it does not have to physically change anything, where has writing does.

like i said it's a complicated process for such a simple question, but if you want to know more about data transfer rates, and throughput i think you'll be interested in these links.

http://en.wikipedia....wiki/Throughput

http://en.wikipedia....work_throughput

http://en.wikipedia.org/wiki/Bit_rate

http://en.wikipedia....Hard_disk_drive

http://en.wikipedia..../DVD_recordable

http://en.wikipedia....lid-state_drive

http://en.wikipedia....USB_flash_drive

[vijay.gupta]

Sorry terry for the query but what do you refer to by physical memory: ram or hard drive? I wil go through the links, however, can you please answer 2 and 3 points in my post above?

[vijay.gupta]

Sorry, you have already answered 3rd point, can you please clarify 2nd point?

[terry1966]

when it starts transferring data from one place to another the first thing it does is start reading from the file you want moved and places that data into it's ram(memory) this is where the transfer rate is very high. before it writes the data to where you want it copied/moved, this is where the transfer rate drops to a steady rate which is limited to the speed it can write to the medium type you are putting the data on. with a hard drive yes the pc will write the data from it's own ram into the hard drives cache(if it has any) again very high transfer rate so the drive can write it to platter again this is where data slows down to a steady rate governed by the speed it can write the data. while the pc goes and fetches more data. flash drives don't have any cache memory tho but the pc will still read the data from the file into it's ram before writing it to the flash drive. so data transfer is high to start with then slows down to the rate the flash drive can be written too. hope that made sense.. didn't to me so needed to edit it some.

Edited by terry1966, 12 October 2010 - 08:48 PM.

[vijay.gupta]

Thanks Terry!! Now, I got it now.....I just tried copying 3 GB file from D: to C: and the speed decreases to a nearly steady speed from a start up high speed. It got resolved but there is similar query: I also tried copying two 3 GB files from CD in my CD Drive to my desktop and the opposite behavior happens. It started with less than 1 MB/sec and then goes to nearly steady speed of more than 5 MB/sec. What is the explanation for this?

Edited by vijay.gupta, 14 October 2010 - 03:52 AM.

[terry1966]

when coping to/from a dvd/cd the disc always needs to start rotating, so it starts off with a slow spin speed of the disc and keeps increasing the rotation speed until it reaches it's maximum read/write speeds, so just like a car it will start off slow and increase speed until it's at it max.

Edited by terry1966, 14 October 2010 - 02:31 PM.

[vijay.gupta]

Thanks Terry for the information!!

[terry1966]

no probs, might not be 100% accurate but i hope it gives you some idea of how things work, transfer speeds depend on a lot of variables, not least where on a platter the data is being written and speed of rotation, how your transferring the data, usb, esata, firewire, etc

[vijay.gupta]

My hard drive is internal to the laptop so it may be sata i think Yes, it give me an idea and that is what i want. Thanks once again Terry!!