Large file copying speed. Why does it slow down?

There is a copy buffer - stuff gets read into memory from the source storage, then written to the target storage, which is checked against the buffer in memory for errors, then the next block of data is read, so at first about 2x the final rate is being transferred, but when the buffer is reading new stuff and checking the last block that has been transferred simultaneously, it has only ~half of the initial rate.

The data copy rate indicator shows the amount of data stored and checked against time - at the end of the operation, it seems to do nothing for a while as data already written to the target is checked against a buffer before the process shuts down.

The reading and writing from memory is the fast part of the operation, but the storage read and write operations are the slowest parts of the operation.

Fafhrd said:

There is a copy buffer - stuff gets read into memory from the source storage, then written to the target storage, which is checked against the buffer in memory for errors, then the next block of data is read, so at first about 2x the final rate is being transferred, but when the buffer is reading new stuff and checking the last block that has been transferred simultaneously, it has only ~half of the initial rate.

The data copy rate indicator shows the amount of data stored and checked against time - at the end of the operation, it seems to do nothing for a while as data already written to the target is checked against a buffer before the process shuts down.

The reading and writing from memory is the fast part of the operation, but the storage read and write operations are the slowest parts of the operation.

Yeah - it is copying to RAM buffer at that speed but can only write to disk at its maximum speed. Once buffer is full, it can only copy as fast as it can write i.e. buffer empties at rate of writing to drive.

That initial burst is an illusion really.

thoatswold said:

I'm surprised you recommend RoboCopy. I've found RichCopy much better, and Microsoft themselves made it available as an improvement over RoboCopy.

Things did change after that last post 3 years ago. One thing about computing and it's need for precision, for better or worse, is the constant change and users having to deal with it.

Just looked for RichCopy, has been discontinued, latest download available from MajorGeeks is

Microsoft RichCopy 4.0.211

Author: Microsoft Corp. Date: 10/14/2009 11:40 AM Size: 5.79 MB License: Freeware Requires: Win 10 / 8 / 7 / Vista / XP